Could it always have looked 13.7 billion years old?

August 24, 2011 3:34 AM

Why does the observed fact that the universe is expanding necessarily imply a Big Bang?

If the rate of expansion is linear then sure, run it backwards far enough and we end up with a universe of radius zero. But there are lots of curves (e.g. exponentials) that approach zero asymptotically rather than actually hitting it; so what good reasons are there for asserting that the observed fact of expansion does in fact imply zero radius at some past time? Has anybody attempted to fit the relatively recent observations that universal expansion is in fact accelerating to an exponential or similar curve? Would positing an exponentially expanding universe of infinite age remove the need for an inflationary period?

School me.

posted by flabdablet to science & nature (16 answers total)

No doubt a proper physicist will swoop in at some point, but a couple of observations in the meantime.
- there's plenty of evidence that the universe was once small and very hot - microwave background for instance, or the distribution of light elements both fit that kind of model.
- your question sort of implies you're wondering about whether there was actually a point where the universe actually had zero size - ie a physical singularity. I think the answer to that is "we don't know" because below a certain size you have to come up with a theory of quantum gravity and we don't have that yet.
posted by crocomancer at 3:41 AM on August 24, 2011

there's plenty of evidence that the universe was once small and very hot

Do we have evidence for how long that was the case? An exponential expansion model would make the small, hot regime last much, much longer than it would have done with linear expansion.
posted by flabdablet at 3:57 AM on August 24, 2011

Well, for one thing, no one's really suggesting that the universe has existed for an infinite amount of time. Current calculations put the "age" of the universe at 13.7 billion years. So I'm not sure it matters whether there was ever a singularity, because that exponential curve is going to have to be one wicked curve to reach the universe's current diameter of 98 billion light years even in that timeframe. So even under your question, there still has to be some pretty amazing event that change the behavior of the universe from being relatively static in a small, hot, state, to rapidly expanding in a much cooler state. It may not be the emergence of the universe from nothingness, but it's something almost as significant.
posted by valkyryn at 4:42 AM on August 24, 2011

No actual physicists yet. Okay. I'm a bit out of my depth now, but I think your idea certainly has testable consequences. The ones that spring to mind are:
- does exponential expansion fit with general relativity predictions at least as far back as we can confidently use them (which is pretty far back I think)
- if you don't have an inflationary period (which I think definitely contradicts what you're suggesting) what's your solution to the problems inflation was proposed to solve (eg flatness, isotropy).

Can't help you with the answers to those, but hopefully that suggests the areas where you'd find evidence one way or the other.
posted by crocomancer at 4:46 AM on August 24, 2011

Physicist here, though I haven't finished my coffee yet so I apologize for any incoherence below.

It really boils down to "that's how the equations work". Basically, the Einstein equation tells you how the curvature of spacetime responds to the matter and energy in it. If we use these equations to "run the clock backwards", and we assume that Einstein's equation is correct and all of the matter and energy in the Universe back then was behaving the way it does now, then we end up with a "Big Bang" singularity at some earlier point in time. An analogy to this would be something like dropping a ball from your hand and saying, "Well, sure it starts out moving downwards, but how do I know that it actually hits the ground? Maybe it slows down and stops five inches above the ground." You can mock up a mathematical description of a world in which this happens, but that mathematical description doesn't actually match the real world.

That said, there's a lot more untested assumptions that go into the statement that "there was a Big Bang" than the statement that "the ball will hit the floor". Maybe Einstein's equations are subtly wrong in some way, in the same way that Newton's equations of gravitation aren't completely accurate. Maybe there was some other weird kind of matter or energy running around in the early universe that we don't know about, and that affected the expansion of the early Universe (inflation, mentioned above by crocomancer, is basically a fudge of this type.) The fact remains, though, that the model we have now — a Universe with matter, dark energy, and an admittedly mysterious inflationary phase in its infancy — is pretty damn simple and yet does a remarkably good job of explaining our observations. Short of some compelling reason to introduce another agent to "eliminate" the Big Bang (by which I mean data that can't be explained in our current model), we might as well make our peace with the notion.

Do we have evidence for how long that was the case? An exponential expansion model would make the small, hot regime last much, much longer than it would have done with linear expansion.

It's not like there was a clock or a video camera counting off time, no; our knowledge is, as I noted above, predicated on Einstein's equations being correct and there not being any other sources of matter or energy to bollix up the works.
posted by Johnny Assay at 5:15 AM on August 24, 2011 [2 favorites]

Inflation isn't consistent with the Einstein equation, is it?
posted by flabdablet at 5:41 AM on August 24, 2011

It's definitely true that certain possible cosmologies in general relativity don't have a Big Bang, but they're excluded by our observations of the universe - how much stuff is in it, and how fast it's expanding. This diagram shows current observational constraints on the kind of universe we live in - that grey region up at the top left which is outside those bounds is the sort of universe that goes asymptotically close to zero as you look back but doesn't hit it. Everything else in that diagram goes through zero.

To get the universes like you're suggesting you basically need way more dark energy than we have in ours.

Now that's all assuming standard general relativity. You might of course have funky physics happening in early times which means you never go to zero - ekpyrotic universes are a notable example of this kind of thing.
posted by edd at 5:43 AM on August 24, 2011 [2 favorites]

Inflation is consistent with general relativity - it just posits an unusual kind of energy to drive it. GR doesn't lay down particularly hard rules on what ingredients you put into your universe - it just tells you how gravity works with them.
posted by edd at 5:44 AM on August 24, 2011

Actually turns out I was slightly wrong - the grey area has bouncing universes rather than ones with exponential expansion - this page has a handy widget for plotting universe histories in that plane (but note the x and y-axes are switched, dark energy runs along the x-axis and matter on the y).
posted by edd at 5:53 AM on August 24, 2011

Simon Singh has a really good book titled Big Bang that does a good job explaining how it became the accepted scientific theory. If you are interested in the subject, I highly recommend it.
posted by smackfu at 5:59 AM on August 24, 2011

See also the Lambda-CDM model

The three big inputs into a cosmological model for our universe are:

The Matter Density (Omega-M) including the amount of visible matter and dark matter.

The Curvature (Omega-K), our universe being nearly Flat.

The Vacuum Energy (Omega-Lambda), that is Dark Energy and the size of the Cosmological Constant.

It is based on the above, all of which can be measured, that we arrive at the Lambda-CDM model.
posted by vacapinta at 6:38 AM on August 24, 2011

The big bang theory doesn't actually tell us whether the universe was actually a point or not. The big bang theory says that the universe at some point was much, much smaller, and was expanding really quite fast. We can extrapolate back an amazing amount, and make some educated guesses, but at some point we just don't know how physics behaves with so much matter/energy crammed into so small a space.

The point exploding into everything idea explains the general idea of the big bang theory without confusing the hell out of everyone, though.
posted by Zalzidrax at 4:43 PM on August 24, 2011

what good reasons are there for asserting that the observed fact of expansion does in fact imply zero radius at some past time?

Big Bang Theory does not say this, as Zalzidrax pointed out. Please see

http://www.talkorigins.org/faqs/astronomy/bigbang.html#misconceptions

Would positing an exponentially expanding universe of infinite age remove the need for an inflationary period?

I'm not sure I understand your question, since inflation *is* an early period of exponential expansion. I think you are using the term "exponential expansion" in a non-standard way . . . and maybe that's confusing you and others.

Do we have evidence for how long that was the case? An exponential expansion model would make the small, hot regime last much, much longer than it would have done with linear expansion.

In the standard use of "exponential expansion", your second sentence is not really true. But to answer the question in your first sentence, yes, we have evidence for how long the Universe was small and hot. We can make observations that tell us the primordial (before being altered within stars) abundances of elements such as Hydrogen, Helium, and Lithium, as crocomancer noted. These elements can be created only under specific conditions, so their observed primordial abundances constrain the time evolution of the pressure and density of the very early (less than 1/2 hour old) Universe. Turns out they match Big Bang Theory really well. You can look up Big Bang Nucleosynthesis or Primordial Nucleosynthesis for more information.

It really boils down to "that's how the equations work".

As an observational astronomer, I'd have to disagree with this. There are lots of observations, such as primordial abundances, the cosmic microwave background, supernovae, and the distribution of large scale structure that say that the universe is currently expanding and that it was smaller and hotter in the past. These are the essential components of Big Bang Theory. The fact that we have equations that describe these elements as accurately as the observations require is bonus, but it's not how we know that these components are there. You can change the theory, but the observations will always exist.

Inflation isn't consistent with the Einstein equation, is it?

There are many solutions to Einstein's equation, depending on your input assumptions. I believe inflation is consistent.
posted by pizzazz at 11:15 AM on August 26, 2011

Thanks, pizzazz, that's really helpful.

About this para from your talkorigins link:

Another cosmologist, the German Rudolf Kippenhahn, wrote the following in his book "Kosmologie fuer die Westentasche" ("cosmology for the pocket"): "There is also the widespread mistaken belief that, according to Hubble's law, the Big Bang began at one certain point in space. For example: At one point, an explosion happened, and from that an explosion cloud travelled into empty space, like an explosion on earth, and the matter in it thins out into greater areas of space more and more. No, Hubble's law only says that matter was more dense everywhere at an earlier time, and that it thins out over time because everything flows away from each other." In a footnote, he added: "In popular science presentations, often early phases of the universe are mentioned as 'at the time when the universe was as big as an apple' or 'as a pea'. What is meant there is in general the epoch in which not the whole, but only the part of the universe which is observable today had these sizes." (pp. 46, 47; FAQ author's translation, all emphasizes in original)

If "What is meant there is in general the epoch in which not the whole, but only the part of the universe which is observable today had these sizes" is right, then I've just had a longstanding misconception wiped out and lost almost all intuitive discomfort with BBT, for which I thank both Kippenhahn and you. Presumably it follows that the oft-quoted "mass of the universe" also refers only to that portion within the Hubble radius. Is that right? If so, I'd be a totally happy camper.
posted by flabdablet at 5:10 AM on August 29, 2011

flabdablet, the short answer to your question is yes, probably. I added the "probably" because I don't often hear astronomers talk about the "mass of the universe". Usually we talk about the matter density (mass per unit volume). That refers to the observable universe. Presumably somebody is multiplying this density by a volume to get the "mass of the universe" that you have heard of. And presumably this volume is the volume of the observable universe. If they did that, the result would apply only to the observable universe. I can't see how someone could quote the mass of the universe in any other way. But as you say, school me!
posted by pizzazz at 4:11 PM on August 29, 2011

I don't often hear astronomers talk about the "mass of the universe".

New Scientist, on the other hand, mentions it frequently :-)

Thanks, pizzazz.
posted by flabdablet at 4:36 PM on August 29, 2011