Why are we so sure the universe existed in a point 13.7 billion years ago?

[PhyPsy]

I've given this a lot of thought the last couple days. I look at the Hubble equation [itex]v=H_0 d[/itex], and I don't see how this necessitates everything existing in a single point at some time in the past. Certainly, everything was really close together 13.7 billion years ago, but not in an infinitely small volume.

[itex]1/H_0[/itex] (Hubble constant is in km/s/Mpc) does not give you the age of the universe (assuming the Hubble constant is constant through time). It gives you the amount of time it takes to travel a Megaparsec going at [itex]H_0[/itex] km/s. If you calculate the amount of time it takes to travel a Megaparsec going 71 km/s, it is indeed around 13.78 billion years. However, any object currently a Megaparsec from the Earth was not always receding at 71 km/s relative to the Earth, as you can see from the Hubble equation. As you go back in time and the object gets closer to the Earth, the velocity decreases. The way I see it, d=0 acts like an asymptote. The object approaches Earth the further back in time you go, but the distance is always non-zero, no matter how far back you go.

Another way to look at it is to see what happens when you apply the case of everything existing in a single point to the Hubble equation. The distance between any object is 0, so the velocity is also 0. If the velocity is 0, nothing moves, and the distance remains 0 for all time. Nothing ever expands! Something magical must have happened for these objects to separate before the expanding could begin.

It seems more logical to conclude that the universe has been expanding for an infinite amount of time. It's just that before 13.7 billion years ago, everything was so condensed that matter could not exist as we know it exists now.

 

[phinds]

I don't see how this necessitates everything existing in a single point at some time in the past.

Uh ... who fed you that nonsense? It was never a point it was just smaller than it is today, and VERY much hotter and more dense.

 

[cristo]

I've given this a lot of thought the last couple days. I look at the Hubble equation [itex]v=H_0 d[/itex], and I don't see how this necessitates everything existing in a single point at some time in the past. Certainly, everything was really close together 13.7 billion years ago, but not in an infinitely small volume.

To put it more politely than the previous poster, this is not true, and is in fact a common misconception. The standard cosmological model, or 'big bang' model, simply states that the universe was once much hotter, denser and smaller than it is today, and that is has expanded from such a state.

If you extrapolate this backwards you arrive with what's called a 'singularity': a point of infinite density and zero volume. However, this is simply a mathematical issue, telling us that our model is not valid at such a time. This is no surprise, since the cosmological model is governed by general relativity, yet when the universe gets very small and hot, we would expect quantum mechanics to be important. When we have a theory of quantum gravity, we expect it to deal with this singularity.

 

[marcus]

I've given this a lot of thought the last couple days. I look at the Hubble equation [itex]v=H_0 d[/itex], and I don't see how this necessitates everything existing in a single point at some time in the past. Certainly, everything was really close together 13.7 billion years ago, but not in an infinitely small volume.
...

Ordinary mainstream cosmologists do not think of the universe being in an "infinitely small volume". That is, I think, more what you hear on television, or is suggested by flashy visuals being fed to the public.

So you are right, in a sense, to challenge the "infinitely small volume" idea. The professionals would agree with you that it is a misconception.

But we don't know what conditions were at the very start of expansion. There are several ideas that have been proposed. They need to be tested observationally. It would be premature to jump to the conclusion that one or the other is right. I would guess that among the various ideas for cosmic models that avoid the infinite-density failure (i.e. that have no "singularity" breakdown) currently more people are studying "bounce" cosmologies than any other model. that does not mean it's right, just that it is currently the most-studied alternative.

The Hubble expansion rate has changed enormously over the history of expansion.
You are right, if you question its constancy! It has definitely not been a constant, over time.
The present value of round 70.4 km/s per Mpc corresponds to distances taking
139 million years to grow by 1%.

So if you watch some distance, between widely separated galaxies, for a million years you can expect to see it increase by 1/139 of one percent.

that fractional or percentage growth rate has been much larger in the past. The 1/139 number is expected to continue declining and level out at around 1/163.

 

[PhyPsy]

I've read about theories that state not only did matter as we understand it not exist before 13.7 billion years ago, but spacetime itself did not exist before then. I understand that GR posits a singularity at that time in history, but does this mean spacetime itself did not exist before then? I find it easier to believe that spacetime existed before then and matter simply expanded enough to finally be able to become how it is known now.

Uh ... who fed you that nonsense? It was never a point it was just smaller than it is today, and VERY much hotter and more dense.

I suppose I ignored other things that would lead one to think the universe existed in a point at one time, such as the singularity predicted by GR. However, there are definitely people who have stated the universe existed in an infinitely small volume. Hawking, for example, said that the big bang singularity is a point of no volume from which everything begins to expand. Well, Hubble's law seems to contradict there ever being a single point in which everything existed.

 

[marcus]

GR (formulated in 1915) is rather old classical (pre-quantum) theory and the fact that it develops a singularity is generally viewed as a symptom that it's wrong or has something missing. A quantum version might fix that. Several quantum theories of how geometry behaves are being studied and in at least one what's been found is that gravity in effect becomes repellent at extreme density. the theory predicts a bounce.

IOW collapse CAN'T be total and complete. When a region collapses to sufficiently high density gravity stops attracting and starts repelling. Quantized geometry resists infinite density.

In the recent MG13 meeting at Stockholm in July there was a special session devoted to "Nonsingular" cosmology models in general, mostly they are of bounce type. And there were also 2 or 3 other sessions devoted specifically to Loop, the general theory and the cosmology, which is one of those where you get a that kind of cosmology.

With such quantized geometry, if you follow our universe's expansion back far enough, where you reach high enough density, then the equations show a prior contracting phase.
So that's one possibility: extrapolating back in time with a model of gravity that bounces rather than suffering a singularity, you see an ordinary universe like ours, with ordinary matter etc., that is collapsing instead of expanding---and when a critical density is reached, bounces and begins the expansion that we do in fact witness.

I'll get a link for the Stockholm MG meeting. It is one of the biggest international conferences where they discuss GR , quantum GR, cosmology, and things like that.
You can check out the relevant sessions. It may not mean much but you can get a general idea of which areas of research are active and what questions people find interesting.

Here's the list of sessions:
http://www.icra.it/mg/mg13/parallel_sessions.htm
Ones to click on, to see the titles of the talks are
CM3
QG1 A
QG1 B
QG4 A
QG4 B
If you find a talk title interesting you can click on it and often get a brief abstract or summary of the talk.

 

[Naty1]

It's easy to be misled when you see comments like "The universe started off no larger than the size of a pea"...when in fact such references allude to a VISIBLE size, not the overall
size.

In other words, if observers existed back then all of them, perhaps many lights years apart, would have made the same observation. In fact the universe was much larger then, perhaps even infinite, just as today we can only observe what is likely an infinitesimally small part of the the total universe.

 

[bcrowell]

We have a FAQ about this: https://www.physicsforums.com/showthread.php?t=506991