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I came to know that the cosmic horizon is the part of universe beyond which we cannot see. So if the light from 13.7 billion years ago is what we can observe, does that mean the universe is older than that and we can't see it?

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No, the oldest light we can see is as old as the Universe$^\dagger$. This is simply because light started traveling at that time, so after one year you'd se light that had been traveling for one light-year, after 1 million years you'd see light that had been traveling for a million light-years, and so on. There is (most likely) more Universe beyond the horizon, but light from those regions just haven't had the time to reach us yet.

You might think that the distance to the cosmic horizon today is 13.7 billion light-years, but because the Universe is expanding, the distance is much larger (in fact 43.6 billion light-years).


$^\dagger$Because the Universe was opaque the first 380,000 years, we can't see all the way, but that's a technical detail.

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  • $\begingroup$ So the age of the universe is still 13.7 Billion years, except that it is expanding so the horizon is increasing? Am i getting it right? $\endgroup$ – quantumbiker Jun 22 at 8:02
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    $\begingroup$ @quantumbiker You're right that the age is 13.7 billion years, and you're right that the Universe is expanding, and you're right that the (distance to the) horizon is increasing. I just wouldn't use the word "except" to join those clauses, because the Universe get older regardless of whether it's expanding or contracting. Moreover, even in a hypothetical, static universe, the distance to the horizon increases as time goes, because light from more and more distant regions reaches you (in this hypothetical case, the distance in light-years would be equal to the age in years). $\endgroup$ – pela Jun 22 at 8:28
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    $\begingroup$ @Edouard No, my answer doesn't take that into account, but those horizons don't really have anything to do with each other. If you're referring to the fact that an observer near an astrophysical event horizon will experience another age of the Universe, that's true, but the term "age of the Universe" is defined for comoving observers, so that doesn't change the conclusion. $\endgroup$ – pela Jun 22 at 21:08
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    $\begingroup$ @Edouard I don't quite follow where you're getting at, but you don't need inflation to have regions that "fall out of" causal contact, you just need accelerated expansion. If expansion continues as we think it will (i.e. if governed by a cosmological constant that doesn't evolve in time), then every point (there aren't gravitationally dominated) will fall out of causal contact. $\endgroup$ – pela Jun 23 at 8:14
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    $\begingroup$ But all of this has nothing to do with what the OP is asking. $\endgroup$ – pela Jun 23 at 8:15
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We didn't determine the age of the universe by looking at what the farthest object is we can see.

Instead, we looked at closer objects and determined their distance, speed and rate of acceleration. When you know those, you can work backwards and find a time when all those galaxies would have been in the same place, which is 13.7 billion years ago.

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  • $\begingroup$ That's true, but the OP's 1st two tags were "cosmology" and "cosmological-horizon", so I think an answer should take account of them, which is why mine still stands. $\endgroup$ – Edouard Jun 24 at 18:13
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The universe might be eternal to the past, as well as to the future. The simplest theory for that is described in "Steady state eternal inflation", formulated by Anthony Aguirre and Steve Gratton, and available free online: It is compatible with observations of the Cosmic Microwave Background, and is centered on a Cauchy surface from which time extends in opposite spatial directions. A similar but somewhat more elaborate cosmology, by Carroll and Chen, is analyzed, together with the Aguirre-Gratton one, by Vilenkin, in his online paper "Arrows of time and the beginning of the universe".

The Borde-Guth-Vilenkin Theorem is usually construed as preventing the "past eternality" of inflation by requiring a balancing of universal expansion by universal contraction, but the references in the last (2003) formulation of that theorem by its three authors include a footnote exempting the Aguirre & Gratton theory from its strictures. Also, the BGV Theorem does not rule out a start of the universe at arbitrarily long times before the start of inflation (quasi-exponential expansion), as hypothesized in Martin Bojowald's 2010 book titled "Once Before Time: A Whole Story of the Universe", which utilizes Loop Quantum Cosmology. Some of the "bouncing" cosmologies, of which the Wiki "Big Bounce" provides an overview, also imply past eternality, and substitute a repetition of "bounces" for the "Big Bang".

What these cosmologists, as well as the three mentioned in the last of my comments below, are trying to do is resolve that contradiction of infinite density in the zero volume of the singularity at which General Relativity has been considered to break down: Because densities in "negative time" might cancel densities in"positive time", the densities of the objects duplicated in either version of the temporal dimension might cancel each other, thereby replacing that contradiction with zero density in zero volume, which makes sense.

I believe that a widespread misconception leading to the belief that the universe, or its massive and energetic contents, are finite is the fact that their limitation to some finite amount would provide the simplest explanation for the fact that the sky is dark at night: However, because of the fact that expansion is different from speed or velocity, and is (consequently) NOT limited to a rate no higher than the speed of light, such accelerations as were witnessed by the Supernova 1A observations in the late 1990's are entirely permissible in General Relativity. The simplest explanations do not necessarily provide the best explanation of scientific phenomena.

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  • $\begingroup$ Re my exchange of comments with Pela, I haven't followed my own advice to the OP carefully enough, and have confounded conformal time with comoving time. Technical difficulties with my computer are preventing me from replying in chat, and time constraints prevent elaboration now, but the differences in our views stem from different definitions of "universe": I'm using BGV's, and Pela's using Davis's. I've upvoted his answer, but mine stands: Other participants can upvote either, both, or neither. $\endgroup$ – Edouard Jun 23 at 16:36
  • $\begingroup$ At youtube.com/watch?v=gfYYkC-TO4k, Guth gives a 2018 lecture supporting a past- and future-infinite multiverse based on the possibility of entropy being infinite, with details being worked out in collaboration with Sean Carroll. As in BGV's exemption of Aguirre's past- and future-infinite proposal, the BGV Theorem would apply separately. in each of the two directions outward from a Cauchy surface. Both of these theories resemble the mathematician Barbour's "Janus universe". The astronomical facts detailed by Pela would apply in whichever of the two halves we find ourselves. $\endgroup$ – Edouard Jun 24 at 17:48

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