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Evidence record

Where does the observable universe end?

Every sentence on the exploration belongs to at least one claim below. Each says what kind of statement it is, what it rests on, what the evidence does not establish, and how far it was checked. Nothing here is a score, and none of it is a verdict on a belief or a theory.

Last checked 24 September 2026.

  • Source checked: 45
  • A judgement, not a fact: 9
  • Presented as open: 2

If something here is wrong, misquoted, out of date or unfair, write to support@unknownoceans.com and give the claim number, which does not change.

What the question takes for granted

A question with a definite answer in the standard model: the observable universe ends at a horizon set by the universe’s age and expansion history. What lies beyond is the open part, and the page treats it as unreachable in principle.

  • The observable universe ends somewhere

    Established

    It does, at the particle horizon, because the universe has a finite age. The end is a limit on observation, not a boundary in space.

  • Where it ends is where the universe ends

    Open

    The page rejects this: the horizon is centred on each observer. Whether the universe as a whole is finite is not known; searches for wrap-around set only a floor.

  • The end can be given as one number

    Contested

    The 46 billion light year radius is a model value, and the true particle horizon depends on how long inflation lasted, if there was inflation.

How the question is labelled

Claim: The far wall

Kind
Framing. Signposting. It asserts nothing a source could check.

Claim: The horizon

Kind
Framing. Signposting. It asserts nothing a source could check.

The question

Claim: There is a distance beyond which nothing can have reached us yet.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 3 · checked 24 September 2026 · AI-assisted audit

Claim: It is not the edge of the universe,

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from the horizon being a sphere centred on each observer, and from no observed boundary or wrap-around (claims 18 and 59).

Claim 4 · checked 24 September 2026 · AI-assisted audit

Claim: and it is not where the light started.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

The horizon is the present distance of the material whose light is arriving now; the light set out when that material was much closer.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 5 · checked 24 September 2026 · AI-assisted audit

Claim: The universe has a finite age, so only light from within a finite distance has had time to arrive.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck 2018 results. VI. Cosmological parameterssection 8, Conclusions

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: the 6-parameter ΛCDM model provides an astonishingly accurate description of the Universe from times prior to 380 000 years after the Big Bang, defining the last-scattering surface observed via the CMB, to the present day at an age of 13.8 billion years

  • Supports

    Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the UniverseFigure 1 caption

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model

    The source reads: only reaches a finite distance: about 46 Glyr, the current distance to the particle horizon. Currently observable light that has been travelling towards us since the beginning of the universe was emitted from comoving positions that are now 46 Glyr from us.

What this does not establish

The age is inferred within the standard cosmological model, not read off a clock.

Later work looked for

Crossref and arXiv, 2020 on, "age of the universe CMB" and "ACT DR6": the Atacama Cosmology Telescope DR6 with Planck (arXiv:2503.14452, abstract) finds the same base model fits, with no departure from flatness; nothing that changes the age.

Claim 6 · checked 24 September 2026 · AI-assisted audit

Claim: That distance is not fourteen billion light years, because space expanded while the light was in transit,

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

Crossref, 2004 on, "particle horizon radius observable universe" and "cosmological event horizon": nothing peer-reviewed that revises the figure; the Planck 2018 parameters (flat, age 13.8 billion years) are close to the model the paper used, and the page keeps "roughly".

Claim 7 · checked 24 September 2026 · AI-assisted audit

Claim: and the horizon is a limit on observation rather than on existence.

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from the particle horizon being defined by what can be seen, as the paper defines it.

Claim 8 · checked 24 September 2026 · AI-assisted audit

Claim: Look far enough in any direction and you stop seeing things.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What stops sight in practice is the microwave background, which the page takes up in its third section.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 9 · checked 24 September 2026 · AI-assisted audit

Claim: Not because they are not there, but because their light has not reached us.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Corrected

24 September 2026. Previously: “Not because they are not there, but because their light is still on its way.” "Still on its way" implies the light will arrive. In an accelerating universe some of it never will, which the page says itself in claim 49; Davis and Lineweaver support only that it has not arrived.

Claim 10 · checked 24 September 2026 · AI-assisted audit

Claim: That limit has a name and a number,

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 11 · checked 24 September 2026 · AI-assisted audit

Claim: and many popular statements of the number are wrong in an interesting way.

Kind
What scholars argue. A report of what specialists argue, or where they disagree.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

The paper collects misleading statements from popular books, textbooks and the research literature; it does not count what share of all popular statements are wrong.

Corrected

24 September 2026. Previously: “That limit has a name and a number, and almost every popular statement of the number is wrong in an interesting way.” "Almost every" is a proportion no source measures. Davis and Lineweaver document "numerous" misleading statements, which carries "many".

Claim 12 · checked 24 September 2026 · AI-assisted audit

What is measured

Claim: What is measured

Kind
Framing. Signposting. It asserts nothing a source could check.

A finite age makes a finite view

Claim: A finite age makes a finite view

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 14 · checked 24 September 2026 · AI-assisted audit

Claim: The cosmic microwave background gives an age of about 13.8 billion years, inferred from the pattern of hot and cold spots in the oldest light there is.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck 2018 results. VI. Cosmological parametersTable 2, base-ΛCDM parameters

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: Age [Gyr] . . . . . . . . . 13 .7971 13 .797± 0.023

  • Supports

    Planck 2018 results. VI. Cosmological parameterssection 8, Conclusions

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: the 6-parameter ΛCDM model provides an astonishingly accurate description of the Universe from times prior to 380 000 years after the Big Bang, defining the last-scattering surface observed via the CMB, to the present day at an age of 13.8 billion years

  • Supports

    Planck 2018 results. VI. Cosmological parametersabstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: We present cosmological parameter results from the final full-mission Planck measurements of the cosmic microwave background (CMB) ... combining information from the temperature and polarization maps and the lensing reconstruction

  • Supports

    Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the Universesection 3.3

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model

    The source reads: Our effective particle horizon is the cosmic microwave background (CMB), at redshift z ∼ 1100, because we cannot see beyond the surface of last scattering.

What this does not establish

An inference under the six-parameter standard model, which Planck fits to the maps; a different model would give a somewhat different age.

Later work looked for

Crossref and arXiv, 2020 on, "age of the universe CMB" and "ACT DR6": the Atacama Cosmology Telescope DR6 with Planck (arXiv:2503.14452, abstract) finds the same base model fits, with no departure from flatness; nothing that changes the age.

Corrected

24 September 2026. Previously: “measured from the pattern of hot and cold spots in the oldest light there is.” Planck derives the age by fitting the standard model to the maps; its own tables list it among derived parameters. "Inferred" says what kind of number it is.

Claim 15 · checked 24 September 2026 · AI-assisted audit

Claim: Light from an object further away than light could travel in that time has not arrived.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

"Further away" here is in the expanding-universe sense the next section explains, which is why the distance is not 13.8 billion light years.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 16 · checked 24 September 2026 · AI-assisted audit

Claim: That sphere is called the particle horizon,

Kind
About a word. What a word in the original language means, or how it is translated.
Review
Source checked. The cited passage was read, and it says this.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 17 · checked 24 September 2026 · AI-assisted audit

Claim: and it is centred on the observer, which means every observer has a different one.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 18 · checked 24 September 2026 · AI-assisted audit

The number is not 13.8 billion light years

Claim: The number is not 13.8 billion light years

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

Crossref, 2004 on, "particle horizon radius observable universe" and "cosmological event horizon": nothing peer-reviewed that revises the figure; the Planck 2018 parameters (flat, age 13.8 billion years) are close to the model the paper used, and the page keeps "roughly".

Claim 19 · checked 24 September 2026 · AI-assisted audit

Claim: The photons arriving now from the most distant material we can see have been travelling for almost the whole age of the universe.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck 2018 results. VI. Cosmological parameterssection 8, Conclusions

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: the 6-parameter ΛCDM model provides an astonishingly accurate description of the Universe from times prior to 380 000 years after the Big Bang, defining the last-scattering surface observed via the CMB, to the present day at an age of 13.8 billion years

  • Supports

    Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the UniverseFigure 1 caption

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model

    The source reads: only reaches a finite distance: about 46 Glyr, the current distance to the particle horizon. Currently observable light that has been travelling towards us since the beginning of the universe was emitted from comoving positions that are now 46 Glyr from us.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 20 · checked 24 September 2026 · AI-assisted audit

Claim: While they travelled, the space between us and their source kept expanding.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 21 · checked 24 September 2026 · AI-assisted audit

Claim: So the source is now much further away than the distance the light covered: roughly 46 billion light years rather than 14.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

The 46 is computed in the paper’s model, with matter and dark energy at 0.3 and 0.7 of the critical density, an age of 13.5 billion years, and no inflation. The paper notes that the true distance depends on how long inflation lasted.

Later work looked for

Crossref, 2004 on, "particle horizon radius observable universe" and "cosmological event horizon": nothing peer-reviewed that revises the figure; the Planck 2018 parameters (flat, age 13.8 billion years) are close to the model the paper used, and the page keeps "roughly".

Claim 22 · checked 24 September 2026 · AI-assisted audit

Claim: The radius of the observable universe is a present-day distance to something we are seeing as it was in the past.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 23 · checked 24 September 2026 · AI-assisted audit

Claim: This is also why galaxies can be receding from us faster than light without anything violating relativity.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 24 · checked 24 September 2026 · AI-assisted audit

Claim: Recession from expansion is not motion through space,

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 25 · checked 24 September 2026 · AI-assisted audit

Claim: and a 2004 paper written specifically to clear up these confusions showed that we routinely observe galaxies whose recession has always exceeded the speed of light.

Kind
What scholars argue. A report of what specialists argue, or where they disagree.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

Shown by calculation in the standard model the paper adopts: all galaxies beyond a redshift of about 1.46 recede faster than light in that model.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 26 · checked 24 September 2026 · AI-assisted audit

Claim: Three things the horizon is not

Kind
Framing. Signposting. It asserts nothing a source could check.

Claim: The observable universe is 13.8 billion light years across.

Kind
Framing. Signposting. It asserts nothing a source could check.

Claim: It is about 92 billion light years across.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

Twice the paper’s radius of about 46 billion light years, so about 92, with the same model assumptions (claim 22).

Later work looked for

Crossref, 2004 on, "particle horizon radius observable universe" and "cosmological event horizon": nothing peer-reviewed that revises the figure; the Planck 2018 parameters (flat, age 13.8 billion years) are close to the model the paper used, and the page keeps "roughly".

Corrected

24 September 2026. Previously: “It is about 93 billion light years across.” The cited paper puts the radius at about 46 billion light years; twice that is about 92, not 93.

Claim 29 · checked 24 September 2026 · AI-assisted audit

Claim: The light took 13.8 billion years to arrive, and its source has been carried further away in the meantime.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck 2018 results. VI. Cosmological parameterssection 8, Conclusions

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: the 6-parameter ΛCDM model provides an astonishingly accurate description of the Universe from times prior to 380 000 years after the Big Bang, defining the last-scattering surface observed via the CMB, to the present day at an age of 13.8 billion years

  • Supports

    Expanding Confusion: Common Misconceptions of Cosmological Horizons and the Superluminal Expansion of the UniverseFigure 1 caption

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model

    The source reads: only reaches a finite distance: about 46 Glyr, the current distance to the particle horizon. Currently observable light that has been travelling towards us since the beginning of the universe was emitted from comoving positions that are now 46 Glyr from us.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 30 · checked 24 September 2026 · AI-assisted audit

Claim: The edge of the observable universe is the edge of the universe.

Kind
Framing. Signposting. It asserts nothing a source could check.

Claim: It is a limit on what has had time to reach us.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 32 · checked 24 September 2026 · AI-assisted audit

Claim: There is no reason to think the universe changes character there,

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from the horizon being centred on the observer: it is a different sphere for every observer, so nothing about the universe itself is located on it.

Other readings

The Planck topology paper adds that the simplest uniform model is likely only an approximation far beyond the scale we can see. That is a remark about what lies well outside, not about the horizon.

Claim 33 · checked 24 September 2026 · AI-assisted audit

Claim: and every observer sits at the centre of their own.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 34 · checked 24 September 2026 · AI-assisted audit

Claim: Nothing can recede faster than light.

Kind
Framing. Signposting. It asserts nothing a source could check.

Claim: Nothing can move through space faster than light.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 36 · checked 24 September 2026 · AI-assisted audit

Claim: The expansion of space is not motion through it,

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 37 · checked 24 September 2026 · AI-assisted audit

Claim: and distant galaxies do recede faster.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

Recession velocity here is the rate of change of present distance in general relativity; the redshift beyond which it exceeds light speed depends on the model.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 38 · checked 24 September 2026 · AI-assisted audit

The wall we actually see is closer than the horizon

Claim: The wall we actually see is closer than the horizon

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 39 · checked 24 September 2026 · AI-assisted audit

Claim: For its first 380,000 years the universe was hot enough to be opaque: free electrons scattered light, and no photon travelled far.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck and the cosmic microwave backgroundsection "What is the cosmic microwave background?"

    Cited · Link refuses automated checks, 19 October 2026 · Read in full, 24 September 2026 · Reference work

    The source reads: for roughly the first 380,000 years, the photons were constantly interacting with free electrons, meaning that they could not travel long distances. That means that the early Universe was opaque, like being in fog.

  • Supports

    Planck 2018 results. VI. Cosmological parameterssection 8, Conclusions

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: the 6-parameter ΛCDM model provides an astonishingly accurate description of the Universe from times prior to 380 000 years after the Big Bang, defining the last-scattering surface observed via the CMB, to the present day at an age of 13.8 billion years

What this does not establish

The standard account of the early universe, in which the figure is a model value; ESA gives it as roughly 380,000 years.

Later work looked for

Crossref, 2020 on, "recombination last scattering surface": how the early universe is described by the standard model; nothing that changes it.

Claim 40 · checked 24 September 2026 · AI-assisted audit

Claim: When it cooled enough for atoms to form, light was released and has been travelling ever since.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck and the cosmic microwave backgroundsection "What is the cosmic microwave background?"

    Cited · Link refuses automated checks, 19 October 2026 · Read in full, 24 September 2026 · Reference work

    The source reads: electrons were able to combine with protons to form hydrogen atoms, and the temperature was too low to separate them again. In the absence of free electrons, the photons were able to move unhindered through the Universe: it became transparent.

What this does not establish

The standard model’s account of the early universe, as ESA summarises it.

Later work looked for

Crossref, 2020 on, "recombination last scattering surface": how the early universe is described by the standard model; nothing that changes it.

Claim 41 · checked 24 September 2026 · AI-assisted audit

Claim: That release is the microwave background,

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck and the cosmic microwave backgroundsection "What is the cosmic microwave background?"

    Cited · Link refuses automated checks, 19 October 2026 · Read in full, 24 September 2026 · Reference work

    The source reads: electrons were able to combine with protons to form hydrogen atoms, and the temperature was too low to separate them again. In the absence of free electrons, the photons were able to move unhindered through the Universe: it became transparent.

  • Supports

    Planck 2018 results. VI. Cosmological parameterssection 8, Conclusions

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: the 6-parameter ΛCDM model provides an astonishingly accurate description of the Universe from times prior to 380 000 years after the Big Bang, defining the last-scattering surface observed via the CMB, to the present day at an age of 13.8 billion years

What this does not establish

The standard model’s account of the early universe, as ESA summarises it.

Later work looked for

Crossref, 2020 on, "recombination last scattering surface": how the early universe is described by the standard model; nothing that changes it.

Claim 42 · checked 24 September 2026 · AI-assisted audit

Claim: and it is a surface rather than a place.

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The sources call it the last-scattering surface, and Davis and Lineweaver note it sits at no fixed comoving position: it is a moment seen from here, not a location.

Claim 43 · checked 24 September 2026 · AI-assisted audit

Claim: It is the furthest anyone can see with light of any wavelength, which is why it is the effective particle horizon rather than the true one.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

The source says we cannot see beyond the surface of last scattering; it speaks of seeing, which is to say light.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 44 · checked 24 September 2026 · AI-assisted audit

Claim: The true one lies beyond it, and by how much is hard to say, because it depends on how long inflation lasted, if there was inflation.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

Inflation is itself a hypothesis about the earliest universe; the source treats its duration as unknown.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Corrected

24 September 2026. Previously: “The true one sits a little beyond it.” "A little beyond" holds only in a model with no inflation. Davis and Lineweaver say the current distance to the particle horizon is difficult to say because the duration of inflation is unknown, and that their figure assumes none.

Claim 45 · checked 24 September 2026 · AI-assisted audit

What is moving out of reach

Claim: What is moving out of reach

Kind
Framing. Signposting. It asserts nothing a source could check.

Whether the reachable universe is shrinking

Claim: Whether the reachable universe is shrinking

Kind
Open question. Something the page says is not known.
Review
Presented as open. The page says this is unresolved, and the review found that it is.

Corrected

24 September 2026. Previously: “Whether the horizon is shrinking” The observable universe’s horizon, the particle horizon of the page’s title, does not shrink. What the passage describes is the event horizon: the part of the universe a signal sent now could ever reach us from. The heading now names it.

Claim 47 · checked 24 September 2026 · AI-assisted audit

Claim: Because the expansion is accelerating, the set of objects we could ever receive a signal from is not growing without limit.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

True in models with an event horizon, which the paper says most viable models have; whether ours does depends on how dark energy behaves (claim 51).

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 48 · checked 24 September 2026 · AI-assisted audit

Claim: Galaxies currently visible are crossing a second, closer boundary, the event horizon, beyond which anything they emit from now on will never arrive.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

A description within the general-relativistic model of an expanding universe that the paper adopts; distances and redshifts in it depend on that model’s parameters.

Later work looked for

How the model describes horizons and recession; not a finding later work revises.

Claim 49 · checked 24 September 2026 · AI-assisted audit

Claim: Davis and Lineweaver put the galaxies crossing it now at a redshift of about 1.8.

Kind
What a model says. What a theory or model predicts or describes, which is not the same as what has been observed.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

Computed in the paper’s model, with matter and dark energy at 0.3 and 0.7 of the critical density and a Hubble constant of 70.

Later work looked for

Crossref, 2004 on, "cosmological event horizon redshift": nothing that revises the figure; it depends on the model, and later parameters are close to the one the paper used.

Claim 50 · checked 24 September 2026 · AI-assisted audit

Claim: How firm that is depends on whether the acceleration is a constant of nature or a changing field.

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from the event horizon being a feature of most, not all, models: whether one exists depends on how the expansion behaves in the far future.

Claim 51 · checked 24 September 2026 · AI-assisted audit

Claim: The DESI collaboration reports that its measurements, combined with the microwave background, prefer a dark energy that changes over time at 3.1 sigma, and at 2.8 to 4.2 sigma with supernovae added, depending on the sample.

Kind
Observation. Something measured or seen, as the named source reports it.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

Significances within one parametrisation of evolving dark energy, reported by the collaboration. They are not a measurement of the event horizon.

Later work looked for

arXiv and Crossref, 2025 on, "DESI DR2 evolving dark energy supernova recalibration": the DES-Dovekie recalibration (Popovic and others 2025) takes the 4.2 sigma combination to 3.2; Efstathiou (2025) argues a supernova offset could explain the most discrepant sample. The page now reports both.

Corrected

24 September 2026. Previously: “DESI now prefers an equation of state that evolves over a constant one, at 3.1 sigma against the microwave background and up to 4.2 sigma with supernovae added, so the question is open in a way it was not a few years ago.” The 3.1 is for DESI combined with the microwave background, not a significance "against" it; the significances are attributed to the collaboration, and the supernova range is given whole, because the 4.2 has since been revised (claim 53) and disputed (claim 54).

Claim 52 · checked 24 September 2026 · AI-assisted audit

Claim: The sample that gave 4.2 gives 3.2 after recalibration, which its own team calls a weak preference,

Kind
Observation. Something measured or seen, as the named source reports it.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

The recalibrated figure combines the supernovae with the microwave background and DESI DR2, as the original 4.2 did. "Weak" is the team’s reading of a Bayesian model comparison; the same data give 3.2 sigma as a frequentist significance.

Later work looked for

arXiv and Crossref, 2025 on, "DESI DR2 evolving dark energy supernova recalibration": the DES-Dovekie recalibration (Popovic and others 2025) takes the 4.2 sigma combination to 3.2; Efstathiou (2025) argues a supernova offset could explain the most discrepant sample. The page now reports both.

Corrected

24 September 2026. Previously: “” Added: the Dark Energy Survey team’s own recalibration of the supernova sample behind the 4.2 lowers it to 3.2.

Claim 53 · checked 24 September 2026 · AI-assisted audit

Claim: and George Efstathiou has argued that an offset between supernova samples could bring the most discrepant one into line with a constant.

Kind
What scholars argue. A report of what specialists argue, or where they disagree.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Evolving dark energy or supernovae systematics?abstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Argument

    The source reads: A comparison of SN common to both the DES5Y and Pantheon+ compilations shows evidence for an offset of ~0.04 mag. between low and high redshifts. Systematics of this order can bring the DES5Y sample into good agreement with the Planck LCDM cosmology and Pantheon+.

What this does not establish

The argument was made against DESI’s first-year results with the older supernova sample; it is one researcher’s case, which the Dark Energy Survey team has disputed.

Later work looked for

arXiv and Crossref, 2025 on, "DESI DR2 evolving dark energy supernova recalibration": the DES-Dovekie recalibration (Popovic and others 2025) takes the 4.2 sigma combination to 3.2; Efstathiou (2025) argues a supernova offset could explain the most discrepant sample. The page now reports both.

Corrected

24 September 2026. Previously: “” Added: a published rival reading of the strongest signal, so the page reports the disagreement and who holds each side.

Claim 54 · checked 24 September 2026 · AI-assisted audit

Claim: So the question is open in a way it was not a few years ago.

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from claims 52 to 54: a preference for evolving dark energy at around three sigma, reported by the collaborations and disputed.

  • Supports

    DESI DR2 results. II. Measurements of baryon acoustic oscillations and cosmological constraintsabstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: This solution is preferred over ΛCDM at 3 .1σ for the combination of DESI BAO and CMB data. When also including SNe, the preference for a dynamical dark energy model over ΛCDM ranges from 2.8− 4.2σ depending on which SNe sample is used.

  • Supports

    The Dark Energy Survey Supernova Program: A Reanalysis Of Cosmology Results And Evidence For Evolving Dark Energy With An Updated Type Ia Supernova Calibrationabstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: Our results hold a significance of 3.2σ, reduced from 4.2σ for DES-SN5YR, to reject the null hypothesis that the data are compatible with the cosmological constant. ... Using generally accepted thresholds for model preference, our updated data exhibits only a weak preference for evolving dark energy.

    Notation restored: The sigma symbols restored from the abstract in the arXiv PDF of arXiv:2511.07517v3, read 2026-09-24. The text as extracted reads: Our results hold a significance of $3.2\sigma$, reduced from $4.2\sigma$ for DES-SN5YR, to reject the null hypothesis that the data are compatible with the cosmological constant. ... Using generally accepted thresholds for model preference, our updated data exhibits only a weak preference for evolving dark energy.

Claim 55 · checked 24 September 2026 · AI-assisted audit

What cannot be reached at all

Claim: What cannot be reached at all

Kind
Framing. Signposting. It asserts nothing a source could check.

Claim: Every observer is at the centre of a sphere that is not the centre of anything.

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from claim 18: each observer’s horizon is a sphere around that observer, so no centre of the universe is implied.

Claim 57 · checked 24 September 2026 · AI-assisted audit

How much is outside

Claim: How much is outside

Kind
Open question. Something the page says is not known.
Review
Presented as open. The page says this is unresolved, and the review found that it is.

  • Context

    Planck 2015 results. XVIII. Background geometry and topology of the universesection 5.2

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: Absent a proper prior giving an upper limit on the size of the fundamental domain, there is always infinite parameter volume available for ever-larger fundamental domains.

Claim 58 · checked 24 September 2026 · AI-assisted audit

Claim: Nothing observed puts a boundary on the universe as a whole.

Kind
Observation. Something measured or seen, as the named source reports it.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck 2015 results. XVIII. Background geometry and topology of the universesection 7, Conclusions

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: The results outlined here show no evidence for departures from isotropic and simply-connected models.

  • Supports

    Planck 2018 results. VI. Cosmological parametersabstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: The joint constraint with BAO measurements on spatial curvature is consistent with a flat universe

What this does not establish

An absence of evidence from searches of particular kinds, reported as such in claims 60 to 63.

Later work looked for

Crossref, 2020 on, "cosmic topology limits circle searches": the COMPACT collaboration (Petersen and others 2023, read in full) shows the matched-circle limit binds only loops through our location, and that most flat shapes can be smaller than the diameter of the last scattering surface; the page now says so.

Claim 59 · checked 24 September 2026 · AI-assisted audit

Claim: Planck looked for the matched circles a universe that wraps around on itself would print on the microwave sky and, for the shapes it tested, found no compact topology at any scale below the diameter of the last scattering surface.

Kind
Observation. Something measured or seen, as the named source reports it.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck 2015 results. XVIII. Background geometry and topology of the universeabstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: both via a direct scan for matched circular patterns at the intersections and by an optimal likelihood calculation for specific topologies

  • Supports

    Planck 2015 results. XVIII. Background geometry and topology of the universeabstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: We specialize to flat spaces with cubic toroidal (T3) and slab (T1) topologies, finding that explicit searches for the latter are sensitive to other topologies with antipodal symmetry. These searches yield no detection of a compact topology with a scale below the diameter of the last-scattering surface.

  • Supports

    Cosmic topology. Part I. Limits on orientable Euclidean manifolds from circle searchesabstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Theory · Model

    The source reads: The most generic signature of cosmic topology in the microwave background is pairs of circles with matching temperature and polarization patterns.

What this does not establish

Planck also used a likelihood search alongside the matched circles. Its explicit limits are for a cubic torus and a slab, with the slab search standing in for other shapes with antipodal circles.

Later work looked for

Crossref, 2020 on, "cosmic topology limits circle searches": the COMPACT collaboration (Petersen and others 2023, read in full) shows the matched-circle limit binds only loops through our location, and that most flat shapes can be smaller than the diameter of the last scattering surface; the page now says so.

Corrected

24 September 2026. Previously: “and found no compact topology at any scale below the diameter of the last scattering surface.” Planck’s searches specialised to a cubic torus, a slab and other shapes with antipodal symmetry. Later work (claim 63) shows other shapes escape the limit, so it is stated for the shapes tested.

Claim 60 · checked 24 September 2026 · AI-assisted audit

Claim: That is a floor under the size, not a measurement of it,

Kind
Observation. Something measured or seen, as the named source reports it.
Review
Source checked. The cited passage was read, and it says this.

Later work looked for

Crossref, 2020 on, "cosmic topology limits circle searches": the COMPACT collaboration (Petersen and others 2023, read in full) shows the matched-circle limit binds only loops through our location, and that most flat shapes can be smaller than the diameter of the last scattering surface; the page now says so.

Claim 61 · checked 24 September 2026 · AI-assisted audit

Claim: and the measurements are equally consistent with infinite.

Kind
Observation. Something measured or seen, as the named source reports it.
Review
Source checked. The cited passage was read, and it says this.

  • Supports

    Planck 2015 results. XVIII. Background geometry and topology of the universesection 5.2

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: Absent a proper prior giving an upper limit on the size of the fundamental domain, there is always infinite parameter volume available for ever-larger fundamental domains.

  • Supports

    Planck 2015 results. XVIII. Background geometry and topology of the universesection 7, Conclusions

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: The results outlined here show no evidence for departures from isotropic and simply-connected models.

  • Supports

    Planck 2018 results. VI. Cosmological parametersabstract

    Cited · Link answered, 19 October 2026 · Read in full, 24 September 2026 · Astronomical observation · Measurement

    The source reads: The joint constraint with BAO measurements on spatial curvature is consistent with a flat universe

What this does not establish

The data set no upper limit on the size of a wrapped-around universe, and fit a flat universe that does not wrap around, which would be infinite. They do not show that it is.

Later work looked for

Crossref, 2020 on, "cosmic topology limits circle searches": the COMPACT collaboration (Petersen and others 2023, read in full) shows the matched-circle limit binds only loops through our location, and that most flat shapes can be smaller than the diameter of the last scattering surface; the page now says so.

Claim 62 · checked 24 September 2026 · AI-assisted audit

Claim: Nor is the floor the same for every shape: later work showed that in all but the simplest ones the shortest loop around the universe can miss us and be two to six or more times shorter than that diameter.

Kind
What scholars argue. A report of what specialists argue, or where they disagree.
Review
Source checked. The cited passage was read, and it says this.

What this does not establish

Worked out for the nine possible flat, orientable shapes. What the circle searches bind is the shortest loop through our own location, which must exceed 98.5 per cent of the diameter.

Later work looked for

Crossref, 2020 on, "cosmic topology limits circle searches": the COMPACT collaboration (Petersen and others 2023, read in full) shows the matched-circle limit binds only loops through our location, and that most flat shapes can be smaller than the diameter of the last scattering surface; the page now says so.

Corrected

24 September 2026. Previously: “” Added: the COMPACT collaboration showed the matched-circle limit is weaker than usually read for most flat shapes.

Claim 63 · checked 24 September 2026 · AI-assisted audit

Claim: This is a permanent limit rather than a temporary one.

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

Follows from the event horizon (claim 49): in models that have one, there is a region from which no signal will ever reach us.

What this does not establish

Whether our universe has an event horizon depends on how dark energy behaves (claims 51 to 55).

Claim 64 · checked 24 September 2026 · AI-assisted audit

Claim: A question about what lies beyond every possible signal is not a question that better instruments answer.

Kind
This page’s reading. A judgement the page makes from the evidence. Others read it differently.
Review
A judgement, not a fact. Reviewed for whether it is fairly put and follows from the checked claims. No source can confirm it.

The page’s own conclusion from claim 64.

Claim 65 · checked 24 September 2026 · AI-assisted audit

What was read