Universal Inflation and Age

[stargazer9915]

Is the universe really 13.7 Billion years old or could it be much older? A good question and here is what makes me ask it.

1. It is generally accepted that the universe is expanding in all directions at the rate of approx. 73 km/sec/Mpc (mega parsec).
2. As far as we can tell, the universe is uniform in all directions.
3. The farthest observed galaxy is approx. 13.7? billion light years away making the observable universe 27.4 billion light years wide (which figures to be 8,120 Mpc if my math is correct).
4. At this rate the universe is expanding by nearly 2 light years per year, or twice the distance at which light travels.

Having said all this, I would like to know how all of the above correlates into the current view of the age of the universe. One other thing, please let me know if anything I have said is not correct or if I have missed something important (which I sometimes do).

[Kwalish Kid]

Is the universe really 13.7 Billion years old or could it be much older? A good question and here is what makes me ask it.

1. It is generally accepted that the universe is expanding in all directions at the rate of approx. 73 km/sec/Mpc (mega parsec).

Yep.

2. As far as we can tell, the universe is uniform in all directions.

Yep.

3. The farthest observed galaxy is approx. 13.7? billion light years away making the observable universe 27.4 billion light years wide (which figures to be 8,120 Mpc if my math is correct).

Nope. It's much further than that.

4. At this rate the universe is expanding by nearly 2 light years per year, or twice the distance at which light travels.

I'm not sure how you get this calculation.

Having said all this, I would like to know how all of the above correlates into the current view of the age of the universe. One other thing, please let me know if anything I have said is not correct or if I have missed something important (which I sometimes do).

The age of the universe is given in terms of the inverse of the current expansion. The thinking is: how far can you run that expansion backwards until everything is 0 distance away from everything else. There is some modification to this, as we are very certain that in the distant past, the expansion was much faster than it is now, so the age is less than that determined by simply running back the expansion that we see today.

That said, there is no reason that the universe has to be as old as this process. It is quite possible that this process of expansion began at some point from a universe in some other state. We do not know enough about the very, very early universe to rule this out.

[Ken G]

4. At this rate the universe is expanding by nearly 2 light years per year, or twice the distance at which light travels.

Close, but actually a proper distance of 13.7 billion light years from us now is separating from us at a rate remarkably close to the speed of light, not twice that. In the current thinking, this is something of a coincidence, involving how the extra push from dark energy appears to have more or less completely compensated the extra pull of normal gravity. But more to the point, there is nothing special about the place that is a proper distance of 13.7 LY from us right now-- that's not the place where the CMB is coming from (it comes from gas that is now much farther from us than 13.7 LY, owing to the expansion). It doesn't seem terribly likely that the universe could be much older than 13.7 billion years, though, because we can map out essentially all of its past dynamics, and it all comes together in quite a dramatic way just about 13.7 billion years ago. Of course, with unknowns like dark matter and dark energy swirling around, we cannot really say anything is impossible, but the basic kinematics of the universe over the last 13.7 billion years or so seem pretty clear.

[Ken G]

That said, there is no reason that the universe has to be as old as this process. It is quite possible that this process of expansion began at some point from a universe in some other state. We do not know enough about the very, very early universe to rule this out.

Anything is possible, but using time to describe some putative earlier pre-universe is fraught with peril, like having any scientific way to define what time even means in such a pre-universe. Science should try to be as parsimonious as possible, so until there's some good reason to distinguish "age of the universe" from "proper time in the Big Bang model", I would think we should not try to introduce any such distinction.

[Spaceman Spiff]

-- that's not the place where the CMB is coming from (it comes from gas that is now much farther from us than 13.7 LY, owing to the expansion).

(emphases mine)



I'd like to comment, here.
The purported age of the universe of 13.7 billion years is too often turned into some "distance" equal to 13.7 billion light years that demarcates some (fictitious) limiting physical spatial size of the universe -- and this mistake alone leads to all kinds of other mistakes in thinking about the expanding universe (not to say that this mistake is being made here). On this page there is a brief discussion of two such errors (and there are more). And this explains how the light travel time "distance" produces more smoke and confusion for the novice than it illuminates. See also here for a description of the various measures of distances in cosmology, and here for a discussion of "the" size of the observable universe.

And as for the CMB -- unlike the light of individual galaxies or quasars, this light had its origin everywhere. When we 'point' to the CMB, we point in all directions, because that's what is observed. We are bathed in a virtually homogeneous and isotropic radiation field whose energy density (in locales near ours in space-time) has been greatly degraded by the factor that the metric has changed (i.e., the scale factor 1 + z of about 1090) since the 'event' of matter-radiation decoupling.

IMO a better way to think of this is that the CMB arose from gas that existed everywhere in a much younger universe (~390,000 yrs old; i.e., a distant place in time). Have a look here.

And of course, the invariant quantity is space-time, which is highly curved.

[Ken G]

All that is quite true. Nevertheless, when we point our telescopes in any particular direction, we see CMB that comes from a relatively specific place. That is the place where now resides a particular collection of matter, expected to be very similar to our own local collection of galaxy clusters, which 13.7 billion years ago was in a very different form (a nearly homogeneous gas of mostly ionized hydrogen that was starting to become neutral), and which is now still more or less in that same place. And, that place is much farther than 13.7 billion light years away from us now, in terms of its current proper distance. These statements are valid in "comoving frame" coordinates, meaning that a "place" is identified with the material that resides there (and more or less remains there forever), and the proper distance to that place is the length of an imaginary string of rulers that would all be placed at rest with the local matter at age 13.7 billion years post-bang.

[Kwalish Kid]

Anything is possible, but using time to describe some putative earlier pre-universe is fraught with peril, like having any scientific way to define what time even means in such a pre-universe. Science should try to be as parsimonious as possible, so until there's some good reason to distinguish "age of the universe" from "proper time in the Big Bang model", I would think we should not try to introduce any such distinction.

There is a good reason: we cannot adequately describe the very, very early universe so we do not have a complete account of the history of the universe. We have no reason to suppose that the age of the universe as we know it is the age of the entire universe.

[Ken G]

There is a good reason: we cannot adequately describe the very, very early universe so we do not have a complete account of the history of the universe.

That statement would carry a lot more force if there was any additional understanding of the very, very early universe offered by postulating pre-universes that are testable in any way other than how they are imagined to affect the very, very early universe. In other words, we can make up anything we like and call it an explanation, completely outside the normal standards of scientific testing. That's precisely what intelligent design does.

We have no reason to suppose that the age of the universe as we know it is the age of the entire universe.

Certainly we do: so far there is no scientifically productive alternative that is demonstrably distinct from a form of self delusion, another similarity to intelligent design.

I would say this all comes under the heading of the very subtle issue of, what is an acceptable explanation. Some early peoples felt that placing the Earth on the back of a turtle qualified as an explanation-- we must try very hard not to fall into that same brand of thinking in regard to pre-universes. It is a very important aspect of all of physics that it cannot explain a first cause, it is completely incapable of that under what qualifies as an explanation in physics. So anything treated as a first cause in physics is also treated as an unexplained initial condition, the two concepts are exactly the same. The Big Bang model found a brilliant solution to this paradox-- anchor your first cause in a breakdown in physics itself, so that any question physics cannot answer about it is a question physics cannot be expected to answer about it. What possible advantage could we accrue from eliminating that solution? An eternally aged universe merely swaps one paradox for another, gaining no power over our condition in the process.

[Kwalish Kid]

That statement would carry a lot more force if there was any additional understanding of the very, very early universe offered by postulating pre-universes that are testable in any way other than how they are imagined to affect the very, very early universe. In other words, we can make up anything we like and call it an explanation, completely outside the normal standards of scientific testing. That's precisely what intelligent design does.

But most Big Bang theorist will admit that the standard cosmological model has no understanding of the very, very early universe that is testable in any way. Take PJE Peebles, for example, who is very explicitly clear on this point.

Certainly we do: so far there is no scientifically productive alternative that is demonstrably distinct from a form of self delusion, another similarity to intelligent design.

Why are you talking about alternatives when there is nothing scientifically on the table? There is no theory of the beginning of the universe. There are a number of untestable hypotheses about the beginning of the universe just like there are a number of untestable hypotheses about an infinite history to the current universe.

The standard cosmological model is a theory about the history of the unvierse as we know it, not about the beginning of the universe. Cosmological texts usually spell this out explicitly.

I would say this all comes under the heading of the very subtle issue of, what is an acceptable explanation. Some early peoples felt that placing the Earth on the back of a turtle qualified as an explanation-- we must try very hard not to fall into that same brand of thinking in regard to pre-universes. It is a very important aspect of all of physics that it cannot explain a first cause, it is completely incapable of that under what qualifies as an explanation in physics. So anything treated as a first cause in physics is also treated as an unexplained initial condition, the two concepts are exactly the same. The Big Bang model found a brilliant solution to this paradox-- anchor your first cause in a breakdown in physics itself, so that any question physics cannot answer about it is a question physics cannot be expected to answer about it. What possible advantage could we accrue from eliminating that solution? An eternally aged universe merely swaps one paradox for another, gaining no power over our condition in the process.

While in popular science reporting people like to discuss this breakdown of physics as an explnation, it is far, far from an explanation. The Big Bang theoriests never, as far as I know, offered the initial singularity as an actual explanation. In what physics is able to explain, there is a gap in our ability to describe matter and energy that comes before we get back to the singularity of the expanding Robertson-Walker models, so we have no testable theory that demonstrates there is such a singularity.

[Ken G]

But most Big Bang theorist will admit that the standard cosmological model has no understanding of the very, very early universe that is testable in any way. Take PJE Peebles, for example, who is very explicitly clear on this point.

And I agree with him, the question is, can we get around this basic untestability by postulating untestable pre-universes, or does that just make the situation worse? It's not a sin to try, but my concern is that the efforts so far do little but "sweep under the rug" the basic unknowns. Such rug-sweeping is exactly what myths did for the ancients-- if you don't know the place of the Earth, say it's on the back of a giant turtle. Question answered, everybody's happy-- but it sure ain't science.

Why are you talking about alternatives when there is nothing scientifically on the table?

The alternative to which I refer is the simple recognition that we do not know and are not going to know, because of a fundamental limit in what science can get us to know. The giant turtle is the approach that does not suffer that limit, but sometimes it is better to recognize a limit than to pretend it isn't there.

There is no theory of the beginning of the universe. There are a number of untestable hypotheses about the beginning of the universe just like there are a number of untestable hypotheses about an infinite history to the current universe.

Exactly. Except I would say that "untestable hypothesis" is an oxymoron, ergo the problem. People can keep trying to get around these limitations, that's fine, but they should be under no illusions that it is quite likely they are not doing science. Perhaps it will become science someday, that is their hope, and more power to them-- but they should not claim it is science before it actually is science. We still need to know a giant turtle when we see one.

The standard cosmological model is a theory about the history of the unvierse as we know it, not about the beginning of the universe. Cosmological texts usually spell this out explicitly.

I have made the same statement myself, countless times on this forum.

While in popular science reporting people like to discuss this breakdown of physics as an explnation, it is far, far from an explanation.

I would also not characterize it as an explanation of the origins of the universe, but it is an explanation of why science cannot answer the question of how the universe originated. Physics never deals with first causes, this should not surprise us. It is not just the Big Bang that is a theory of subsequent evolution-- all of physics has that same character, that's just what physics does. To me, it is a cop out to try to escape that fact by trying to cook up an untestable model of a pre-universe that can be infinitely old, simply to push "out of sight" the basic truth that physics does not do first causes.

[Kwalish Kid]

And I agree with him, the question is, can we get around this basic untestability by postulating untestable pre-universes, or does that just make the situation worse? It's not a sin to try, but my concern is that the efforts so far do little but "sweep under the rug" the basic unknowns. Such rug-sweeping is exactly what myths did for the ancients-- if you don't know the place of the Earth, say it's on the back of a giant turtle. Question answered, everybody's happy-- but it sure ain't science.

Your position seems to be that, even though most cosmologists do not claim to know anything beyond a certain point in the past, we should nonetheless claim that there is an actual age of the universe. To me that seems equivalent to claiming that there is a turtle on which the Earth stands.

The alternative to which I refer is the simple recognition that we do not know and are not going to know, because of a fundamental limit in what science can get us to know. The giant turtle is the approach that does not suffer that limit, but sometimes it is better to recognize a limit than to pretend it isn't there.

I don;t understand. You seem to be identifying the "age of the universe" as given in popular accounts of the Big Bang with an actual age of the universe. That seems to be ignoring the actual limits imposed by physics.

Exactly. Except I would say that "untestable hypothesis" is an oxymoron, ergo the problem. People can keep trying to get around these limitations, that's fine, but they should be under no illusions that it is quite likely they are not doing science. Perhaps it will become science someday, that is their hope, and more power to them-- but they should not claim it is science before it actually is science. We still need to know a giant turtle when we see one.

I agree that these currently untestable hypotheses need not be forever untestable. Many of these hypotheses are bound up with M Theory and other proposed successors to General Relativity. Tests of these theories are being devised. But, again, I claim that to ignore that the standard cosmological model doesn't really commit to a beginning of the universe is to engage in turtle thinking.

I would also not characterize it as an explanation of the origins of the universe, but it is an explanation of why science cannot answer the question of how the universe originated. Physics never deals with first causes, this should not surprise us. It is not just the Big Bang that is a theory of subsequent evolution-- all of physics has that same character, that's just what physics does. To me, it is a cop out to try to escape that fact by trying to cook up an untestable model of a pre-universe that can be infinitely old, simply to push "out of sight" the basic truth that physics does not do first causes.

My claims have nothing to do with making claims about the nature of physics. I was pointing out that the "age of the universe" associated wiuth the Big Bang is not established as the actual age of the universe. You seem to want to claim that it is because the alternatives to the claim aren't currently testable while you apparently ignore the fact that claims that the BB-"age of the universe" is the age of the universe is also untestable.

[Cougar]

Your position seems to be that, even though most cosmologists do not claim to know anything beyond a certain point in the past, we should nonetheless claim that there is an actual age of the universe.

Personally, I've started qualifying this by saying 13.7 billion years is the age of the current expansion (the beginning of which we refer to as the "big bang"). Before that, we don't know, and may never know. A standard claim is that time itself came into being at the big bang, but I think more current thinking takes this as rather presumptuous.

[speedfreek]

I tend to think of it like this:

We cannot meaningfully discuss time, when looking backwards past a certain point in the history of the universe, if there is a singularity in the way.

[Jeff Root]

Kwalish Kid, Cougar,

That is exactly what I would have said if I had the necessary brains.
Thank you!

-- Jeff, in Minneapolis

[Ken G]

Your position seems to be that, even though most cosmologists do not claim to know anything beyond a certain point in the past, we should nonetheless claim that there is an actual age of the universe. To me that seems equivalent to claiming that there is a turtle on which the Earth stands.

The difference is that I require a scientific definition of "age". That is it, that's all I ask for. Thus, I stick to the meaning of age that actually does have a scientific definition, cognizant of how precise scientific definitions always need to be. What is your definition of that word?

I don;t understand. You seem to be identifying the "age of the universe" as given in popular accounts of the Big Bang with an actual age of the universe. That seems to be ignoring the actual limits imposed by physics.

I have no idea what you mean by the actual limits imposed by physics. The words I am using all have precise physical meanings, within a particular testable model-- that's the point of words in science. Any other use of scientific-sounding words that do not carry with them any precise meaning in the context of a testable theory is what I mean by a giant turtle. For example, the word "age" conjures the concept of "time", a word with a clear meaning in our universe, but which means....??? in an imaginary "pre-universe."

I agree that these currently untestable hypotheses need not be forever untestable. Many of these hypotheses are bound up with M Theory and other proposed successors to General Relativity. Tests of these theories are being devised. But, again, I claim that to ignore that the standard cosmological model doesn't really commit to a beginning of the universe is to engage in turtle thinking.

Again, the standard cosmological model commits to using words it can actually define. The price it pays is inability to reason about first causes. This is quite a standard tradeoff in physics, I have no idea why cosmology would want to be any different.

[Jeff Root]

There is no good reason to think that time did not exist prior to the Big
Bang, and there is no good reason to think that time was any different
before the Big Bang from the way it was after.

The cosmic expansion, extrapolated backward in time, tells us that about
13.7 billion years ago, all matter was densely packed together. It doesn't
tell us that time began then.

-- Jeff, in Minneapolis

[Ken G]

There is no good reason to think that time did not exist prior to the Big
Bang, and there is no good reason to think that time was any different
before the Big Bang from the way it was after.

As science is not primarily an exercise in philosophy, it is not the purvey of science to decide if there is a "good reason" or not to imagine when time began. In science, time is something you can measure locally, and interpret globally, to make sense of observations. Therefore, the concept of time is perfectly useless in every situation where it involves none of those things. I'm not saying it's impossible to get a concept of pre-universe time to do any of those things, I'm saying no one ever has, and the onus of demonstration falls on the claim, not the absence of the claim. I do not say time is categorically meaningless in a pre-universe, I say that holds until otherwise demonstrated. I await any such demonstration.

Put differently, your argument above reverses scientific logic-- good science does not say "let's use science to decide what must exist, and extrapolate that ontology into all kinds of imaginary spheres that make no contact with what we can test", it says "let's use what we observe to decide what exists, and invent unifying organizational concepts like time to help us make sense of these observations, with no unnecessary extrapolations or extraneous ontologies surrounding the process." Hence, if we only observe evidence of a particular time stream, then time only has scientific meaning in the context of that stream. That is also why I am still waiting to have my earlier challenge met, of someone providing a scientific definition of time that applies to untestable pre-universes.

The cosmic expansion, extrapolated backward in time, tells us that about
13.7 billion years ago, all matter was densely packed together. It doesn't
tell us that time began then.

Actually, it says quite a bit more than that! It says that if you run time backward, you reach a point where the scientific method doesn't work any more. And anyone who claims otherwise needs to demonstrate their claim, instead of merely assuming it, against the perfectly apparent evidence to the contrary.

[Kwalish Kid]

I have no idea what you mean by the actual limits imposed by physics. The words I am using all have precise physical meanings, within a particular testable model-- that's the point of words in science. Any other use of scientific-sounding words that do not carry with them any precise meaning in the context of a testable theory is what I mean by a giant turtle. For example, the word "age" conjures the concept of "time", a word with a clear meaning in our universe, but which means....??? in an imaginary "pre-universe."

But I'm not talking about an imaginary pre-universe, I'm talking about an era of the universe as it is imagined by Big Bang scientists. I'm not talking about the supposed singularity and I'm not talking about before this supposed singularity. I'm talking about how we know that, at a certain point in the past, the energies and temperatures in the universe were much higher than we can reasonably describe with the physics that we have available. This means that we cannot extrapolate back to this singularity that we might actually call the beginning. We have every reason to believe that this era of the universe actually existed, but no way to determine the rules governing spacetime in this era, except to be reasonably sure that there is space and time in this era.

Again, the standard cosmological model commits to using words it can actually define. The price it pays is inability to reason about first causes. This is quite a standard tradeoff in physics, I have no idea why cosmology would want to be any different.

This is not a question of first causes, this is a question of a period of cosmological history that we cannot yet accurately describe.

Actually, it says quite a bit more than that! It says that if you run time backward, you reach a point where the scientific method doesn't work any more. And anyone who claims otherwise needs to demonstrate their claim, instead of merely assuming it, against the perfectly apparent evidence to the contrary.

This is exactly what you are doing. By ignoring that the ability of the standard cosmological model to describe the history of the universe ends significantly before we can extrapolate back to the singularity, you are committing to something that goes well beyond what the science says.

[Ken G]

I'm talking about how we know that, at a certain point in the past, the energies and temperatures in the universe were much higher than we can reasonably describe with the physics that we have available. This means that we cannot extrapolate back to this singularity that we might actually call the beginning.

I completely agree-- fortunately the statement that time has had a meaning in this universe for 13.7 billion years is utterly insensitive to any requirements of thinking about the pseudo-temporal neighborhood of a singularity. I don't begin the timeline at the singularity, I begin it when time begins to have a precise definition that has proven successful at unifying observations.

We have every reason to believe that this era of the universe actually existed, but no way to determine the rules governing spacetime in this era, except to be reasonably sure that there is space and time in this era.

If we have no way to determine the rules governing spacetime in that era, why on Earth would we want to assert that we can be "reasonably sure" there was space and time? The history of science is littered with reasonably sure people making all kinds of crazy assertions that proved to have no basis for reasonable sureness.

This is not a question of first causes, this is a question of a period of cosmological history that we cannot yet accurately describe.

If there is a period of cosmological history we cannot accurately describe using the concept of time, then we will need a new concept. If we can describe it using time, it adds nothing significant to 13.7 billion years.

By ignoring that the ability of the standard cosmological model to describe the history of the universe ends significantly before we can extrapolate back to the singularity, you are committing to something that goes well beyond what the science says.

I am certainly not ignoring that fact, indeed I am basing my entire argument on that fact.

[Kwalish Kid]

If there is a period of cosmological history we cannot accurately describe using the concept of time, then we will need a new concept. If we can describe it using time, it adds nothing significant to 13.7 billion years.

You are incorrect on two points. First, we can describe this era of the universe using time, but we haven't any testable theories of the equations of state of the mass-energy densities of that era. Second, it could be that there is a significant change that lengthens the relationship of these mass-energy densities to the scale factor. As it is the scale factor that determines the history of the universe, beyond a certain point we cannot determine this history, including its duration.

[Ken G]

You are incorrect on two points. First, we can describe this era of the universe using time, but we haven't any testable theories of the equations of state of the mass-energy densities of that era.

What era are you talking about? If you are saying we need testable theories for that era, clearly we do, but what timeframe are you talking about that will add measurably to 13.7 billion years? What era is that, and what testable theories do you have in mind, that are going to change that, in ways that unify observations we can actually make?

Second, it could be that there is a significant change that lengthens the relationship of these mass-energy densities to the scale factor. As it is the scale factor that determines the history of the universe, beyond a certain point we cannot determine this history, including its duration.

That is an entirely different issue, as it involves no "pre-universes", merely updates to our current testable theories. Science constantly updates its understanding, so all scientific knowledge is provisional to our current best theories. Nothing new there. Yet, it is our current best theory that indicates the age of the universe as 13.7 billion years, and that type of age determination is all science is ever capable of doing. If all you are saying is "we don't know everything so any age determination we make could change a century from now, or even tomorrow", then of course I agree-- that statement is always true and will always continue to be true.

[David-M.]

Doesn’t inflation (in a Guth/Linde model) require a random fluctuation of a scalar field (inflaton) over a small region of a pre-existing Universe?