cosmic birthday

morningstar

How is the age of the universe measured? Is it by the distance light has traveled since the big bang? Does that imply a closed universe?

Chalnoth

1. Through model fitting. We have a model for the contents of the universe (matter, radiation, dark matter, dark energy, etc.), and determine the model parameters based upon the data. Extrapolating this model back in the past leads to a singularity in the model. We don't take this singularity seriously: it is obviously an incorrect description of the very early universe. But it serves as a decent-enough zero point. These days, the data are good enough that it doesn't matter that much which particular model you choose for the age of our universe.
2. Unfortunately, there's no way to directly measure the distance light has traveled since the big bang.
3. No, it doesn't imply a closed universe.

morningstar

thanks chalnoth,

although the distance that light has traveled since the big bang cannot be measured, are there any theoretical models that hold the size of the universe- as measured by the expansion of light since the big bang- constant? in a model like that it would seem that everything inside the universe is shrinking.

if all the energy of the universe can be measured within the body of light emitted by the big bang, then it seems that, what we perceive to be, spatial expansion/inflation, is not really making the universe any bigger, it is simply redistributing energy; and making cosmological bodies smaller, in a very real sense.

Chalnoth

Well, sort of. A very common coordinate system to use in cosmology is the comoving coordinate system: coordinates that expand as the universe expands. In comoving coordinates, the coordinate distance between objects tends to not change (except for local motions due to gravitational collapse).

Well, the problem there is that there is no absolute definition for "total energy" for the universe. In fact, overall, energy doesn't even appear to be conserved in an expanding universe. See here for a rather in-depth discussion of energy conservation in General Relativity:
http://math.ucr.edu/home/baez/physic...energy_gr.html

morningstar

great link!

if we are shrinking- relative to the size of the universe and the light that is moving away from us since the big bang- then we must be shrinking at an accelerating rate. could this be the cause of our perception that cosmological bodies are moving away from us? it seems that, relative to the size of the universe, they must be getting closer to us. do comoving coordinates account for that?

Chalnoth

There's no difference between the two descriptions. In the normal description, atoms are stable and the universe is expanding. In the other, atoms are shrinking while the universe is stable. We usually choose the former description because rewriting electricity and magnetism as well as quantum mechanics to take this shrinking into account would be horribly complicated, and would contain a number of terms that don't seem to make sense (e.g. the value of the cosmological constant). But it's useful to use the second description when we don't care about describing the behavior of atoms precisely, as it makes some cosmological calculations easier.

Bear in mind that neither description is more true or less true than the other. They're just different perspectives.

morningstar

aside from the math, if we hold a theoretical edge of the universe constant, and everything inside as shrinking, then isnt the speed of light changing as our rulers literally get smaller to measure it. though I understand that would make our clocks slower, so we wouldn't perceive it as slowing. but the distance that light would cover would be shorter- relative to the size of the universe- over a duration of time that we measured to be the same?

Chronos

We can constrain the age of the universe, but, not its spatial extent. It remains possible the universe was infinite to begin with. Btw, the shrinking universe idea conflicts with an abundance of observational evidence.

morningstar

thanks for the reply chronos- do you have any links or examples of the conflicting observational evidence?

Chalnoth

I don't think it can. It's just a change in coordinates, after all. But it is very difficult in practice to adjust the laws of physics to compensate for this coordinate change.

Chalnoth

Well, no, it doesn't make our clocks slower. But it necessitates rewriting a lot of physics to keep everything consistent. As Chronos notes, however, there's no reason to believe our universe has an edge.

morningstar

is not the measure of light simply a measure of time? if our rulers are shrinking and we measure light to be the same speed then time must be slowing at the same rate. it seems our perception of time is as a fish's perception of water- per chronos' post-script.

Chalnoth

Well, no, the speed of light is the ratio of distance to time. So when you start changing the length scales, then the speed of light changes as well, because the ratio of distance to time changes. If you also changed time, you'd have something completely different going on. It's possible to do, but then that just adds another level of complexity to an already complex problem.

morningstar

Yes, I see what you mean. In our case we hold the ratio of distance to time to be nearly 300,000km/s. But if that 300,000km ruler is a rigid body then it would seem to come up short measuring the light that was emitted only a second prior; because that light emitted only a second prior would take up a larger proportion of the universe than the light emitted presently. So if we think of a clock as a number of periodically predictable occurrences, carried by light, over a distance, I believe our clocks would be speeding up.

Could this be a predictor of radiation, emitted at earlier times in the universe, having longer wavelengths?

Chronos

Everything we think we know about physics pretty much gets tossed out the window if spectral lines vary with respect to the age of the universe. We would, instead of a difficult problem, be faced with one that is well nigh insurmountable.

Chalnoth

Yeah, but making the change to a coordinate system where atoms shrink is really just a coordinate system change. Sure, it makes the calculations for how matter behaves horribly complicated, but it is just a coordinate system change.

morningstar

In this model, it seems we are holding light as fixed in space. If there is any inflation, it would seem to be because space was "flowing" into that initial sphere of light and miniaturizing everything inside of it- from atoms to galaxies. If space is isotropic and homogenous (not spring loaded) then any "increase" in space would yield a uniform and proportional increase between masses, relative to the proportion of space- to the whole, within the body of the first light- between those two masses, down to the most fundamental particles; i.e. an increase in space anywhere yields a proportional increase in space everywhere. So it seems that what we perceive as inflation is also occurring, at a subatomic level, within ourselves, if space is isotropic and homogenous. Also, if we are holding light as static, it would seem that we are shrinking at a rate of c^2- in every direction; which gives me a profound respect for the accelerating rate at which space is moving local subatomic material, along with its previously admitted light/energy (shape defined by relativistic factors), closer together- in order to keep us from flying apart. The logical conclusion of this model seems to be that we- and the entire cosmos that we know- are as an infinitesimally small point- getting smaller- in the center of a static sphere of light that we refer to as the first light of the universe/the first light emitted by the big bang. From a distance, it would appear the entire cosmos is approaching a singularity. Of course, we realize this does not appropriately reflect what we observe. Our clocks are getting faster, not slower, in this model.