# Growth of the universe?

• B
• J251H
The standard model does not say that the universe "grows", it says that on large distance scales all of the matter in the universe is moving apart.

#### J251H

I know nothing about physics, but it would be great to get an answer to my questions of anyone can help:

1) If everything that exists does so within the universe, does it make any sense to say that the universe 'grows'? Against which yardstick is any growth being measured?

2) Would the energy density of the universe be any different now than at the time of the big bang if all the energy were released into free form at the same time?

3) Is it possible that it is the condensation of energy into matter which creates the impression of a growing universe to those of us which live within it? (and that in fact the universe is no bigger than at the time of the big bang)

Let me just address your first question. Contrary to what many people think, it is possible for something which is infinite in extent to "expand". It simply means that the distances between objects is getting larger with time. Imagine your one-dimensional number line, which stretches to infinity in both directions. At time t=0, the distance between the point labeled "1", and the point labeled "2" is one unit. Similarly, the distance between the point "1" and the point "11" is 10 units. At some time later, t=T, the distance between "1" and "2" has grown to 1.1 units, and the distance between the point "1" and "11" has grown to 11 units. The number line is infinite in extent at t=0, and it is infinite in extent at t=T, but it has "expanded". It is in this sense that the universe is expanding. You ask "Against which yardstick is any growth being measured?". The answer is that there are other things, like the sizes of atoms or the wavelengths of light emitted by atoms, which are not expanding with time, and it is against these yardsticks that we measure the expansion.

I guess that means that if all the energy in the universe were to be simultaneously released into free form, there would now be lower energy density than at the time of the big bang (as it would be spread over a larger volume). Is there any way to confirm this though?

J251H said:
I guess that means that if all the energy in the universe were to be simultaneously released into free form, there would now be lower energy density than at the time of the big bang (as it would be spread over a larger volume).
Yes, that's correct.
Is there any way to confirm this though?
Well, this type of assumption is a fundamental part of the standard model of cosmology (the ΛCDM model), which does an extraordinarily good job of explaining our observations. So if this weren't true, many things wouldn't make sense. For example, the Cosmic Microwave Background (CMB) is assumed to have been emitted at a time when the energy density of the universe was ~ 10^9 times higher than it is today, and was comparable to the energy density at the surface of the sun. Since the properties of the CMB are extremely well described by the ΛCDM model (see http://cosmology.berkeley.edu/Education/CosmologyEssays/The_Cosmic_Microwave_Background.html for example), we are quite confident that the fundamental assumptions, like the scaling of energy density, are correct.

J251H said:
If everything that exists does so within the universe, does it make any sense to say that the universe 'grows'?

The standard model of cosmology does not say that the universe "grows" (although pop science descriptions often make it seem that way). It says that, on large distance scales (roughly 100 million light years to a billion light years and larger), all of the matter in the universe is, on average, moving apart.

J251H said:
Would the energy density of the universe be any different now than at the time of the big bang if all the energy were released into free form at the same time?

What does "released into free form" mean?

If you just mean the average density of energy in the universe now as compared to right after the big bang, yes, it is many, many orders of magnitude smaller now.

J251H said:
Is it possible that it is the condensation of energy into matter which creates the impression of a growing universe to those of us which live within it? (and that in fact the universe is no bigger than at the time of the big bang)

I don't know what you mean by "condensation of energy into matter". But I think you are trying to work with a mistaken understanding of what our standard cosmological model actually says.