59

Beyond the fact that the cosmic microwave background (CMB) is a direct prediction of the big bang model, there is the question of how you would produce it in any other way. It is remarkably close to being isotropic and remarkably close to being a blackbody spectrum - i.e. it is almost a perfect blackbody radiation field. A blackbody radiation field is ...


33

Well, suppose that you're in the office and you go to the coffee machine. You notice that there is an incredibly tiny puddle of coffee left in the pot. The pot is almost completely empty, but it's not totally empty either. Why is this? One theory is that it could be a complete coincidence -- the pot was going to have some coffee level or other in it, and ...


22

You are quite correct that we can't see what happened before the CMB (this time is known as recombination) but this is not unusual in Physics. For example we can't see what happens at collisions in the Large Hadron Collider. All we can see is the debris that comes flying out of the collisions. But we understand the physics involved so by measuring the ...


21

The big bang model$^{1}$ is an attempt to explain a host of observations that tell us how the universe evolved from its first fraction of a second onwards. It started off by being grounded in fundamental and rather well understood physics (General Relativity, particle physics and nuclear physics at relatively low energies). The model itself has evolved and ...


20

The rough idea is that under the assumptions contained in the cosmological principle, the application of Einstein's equations leads us to the equation $$d(t) = a(t) \chi$$ where $d(t)$ is called the proper distance and $\chi$ is called the comoving distance between two points in space. $a(t)$ is the time-dependent scale factor, which is by convention set to ...


19

Two issues: 1) Your objection would seem to apply to any explosion. The initial state is hot, the final state is cold. What happens is that the initial state has high entropy density, but small volume. During the explosion volume increases, and entropy density decreases. Overall, the total entropy goes up somewhat, because of non-equilibrium processes (as ...


17

Judging by the various comments and answers I have seen posted so far, it seems to me that the main mistake made by the questioner is to assume that cold things always have lower entropy than hot things. This is wrong because the states counted in entropy measurements are states in position-momentum space (called phase space), not just momentum space. During ...


16

The answer touches upon the concept of Big Bang Nucleosynthesis (BBN), which is excellently explained on a graduate level in Baumann's lecture notes. The key idea is the following: In order to form metals (anything heavier than hydrogen and helium), you need deuterium nuclei. But deuterium nuclei are only formed significantly when the temperature of the ...


15

I'm not too interested in providing an answer from the cosmological point of view. It is clear that the age of the universe derived in that way is model-dependent. The age thus obtained depends on certain assumptions (e.g. that the dark energy density remains constant). I will just add a couple of additional age determination methods that rely on alternative ...


13

The Big Bang model of an expanding Friedmann universe, with a hot and dense early era that cooled and thinned as the universe expanded, predicted the existence, isotropy, and approximate temperature of the cosmic microwave background. So the discovery of this CMB is considered strong confirmatory evidence for that remarkably simple and elegant model. See ...


10

First, you're quite wrong about Bob --- when he returns, he shares Alice's frame, and so agrees with Alice that the universe is 14.8 billion years old --- and that he once spent one billion of those years traveling, during which time he aged only .1 billion years, though this has nothing to do with how long ago the Big Bang occurred. The person you want to ...


9

You are correct, there is no absolute time in the universe. This is as per SR, time is relative. Now to whom is the universe 13.8 billion years old? To a comoving observer with zero comoving velocity (peculiar velocity). The "age of the Universe" of about 14Gyr you frequently hear about is a good approximation for any observer whose peculiar velocity is ...


7

We have the standard model of particle physics, which has been well validated by experiments during the last decades or so, to the point that one can say that it is a convenient encapsulation of all the data. The table of elementary particles which is axiomatic in the theory, has again axiomatically a table with the antiparticles, the theory is symmetric in ...


7

The Big Bang theory does not in itself need to focus on the distinction between matter and radiation. And the Big Bang theory is not a statement about origins in the philosophical sense. Rather it is a statement about the nature of the evolution of the universe from very early times. It holds that that evolution is one in which an initially hot dense state ...


6

Two photon interactions are very improbable. This is because it is a quantum mechanical interaction and controlled by the electromagnetic coupling and the masses entering in the propagators of the charged virtual particles. To get into a thermodynamic equilibrium (classical physics) a large number of scatters should exist so as to apply statistical ...


6

To compute the age of the universe, one must solve the equation: $$\frac{1}{a}\frac{da}{dt} = H_0 \sqrt{\frac{\Omega_{\gamma,0}}{a^4}+\frac{\Omega_{m,0}}{a^3}+\frac{\Omega_{k,0}}{a} +\Omega_{\Lambda,0}}$$ where $\Omega_\gamma$, $\Omega_m$, $\Omega_k$, $\Omega_\Lambda$ are the densities of radiation, matter, curvature, and vacuum energy, and the subscript '0' ...


6

The Big Bang started from a singularity -- which is to say not a physical singularity, which would be an oxymoron. A singularity means a point at which we have no mathematically valid description of physics. General relativity implies nothing at a singularity, except that we need a more comprehensive theory to say what happened. Going back as far as we can, ...


5

It isn't the oldest thing we can see. Most of the hydrogen, helium and deuterium nuclei that are around in the universe now, were created in the time period between a few seconds and about 15 minutes after the big bang. The abundances of these nuclei in the universe is a direct probe of the physical conditions, and time evolution of those conditions, at ...


5

There's no reason for supposing that matter and antimatter should have been produced in equal quantities in the Big Bang. It's simply what is the most "natural" (inverted commas because this isn't well-defined and different people might find it different things more natural). Compare the related question: what is the total net electric charge of the ...


5

This is not a full answer, but I think it will help if you separate out inflation from the rest of the picture. The age of the universe can be estimated in the first instance as the time elapsed since some very early epoch where the temperature was low enough that the Standard Model of particle physics applies to reasonable approximation. This means you can ...


4

Relativity has ways to make time pass arbitrarily slow, but no ways to make it pass arbitrarily fast. So if you are uncomfortable with defining what it means to "move with the universe", there is another way. You can define the age of the universe as the longest possible time any observer can need to get from the big bang to our current position in space-...


4

Interesting question! Let me see if I can shed some light with an analogy. Btw, I shall be referring to John's answer at points. Studying or researching in astronomy is very much like criminal investigation. You have the crime, you search for clues which are used to reconstruct the events of the crime. Here, we have a crime, the construction of the universe,...


4

The strongest motivation for initially equal amounts of matter and anti-matter actually comes not from the standard model but from cosmology - there is a staggeringly large number of photons in the Universe, relative to baryons$^1$. It's possible to arrive at this conclusion a couple of different ways. One is to look at the cosmic microwave background, which ...


4

The real answer is that the big bang singularity isn't part of modern cosmology, in part because of the horizon problem. The big bang model is only valid back to an early era when the scale factor was nonzero; before that, something else happened. Various inflationary models are the most popular, but there are others. What these models have in common is that ...


3

Yes. We can either look for the cosmic neutrino background or for gravitational waves associated with the big bang. Both of these probe the conditions considerably earlier than the the 380,000 years after the big-bang probed by the CMB. Neither of these are fanciful ideas - there are good theoretical reasons to expect both to exist, and in both cases there ...


3

If everything were accelerating towards a point, then this would impose an anisotropy in the redshift-distance relation of galaxies that depended on which direction they were in. It is also not the case that the universal expansion has always accelerated. There is good evidence from observing high redshift $(z>1)$ type Ia supernovae that the universal ...


3

What's called the "age of the universe" would more accurately be called the age of the most recent epoch in the universe's history. That epoch began with the end of inflation, or with the end of whatever noninflationary process created the incredibly uniform expanding quark-gluon plasma that eventually clumped into stars, planets and us. We don't ...


3

No, the combination releases enough energy to heat the surroundings enough that the entropy increases.


3

Before the electrons and protons merged together to form atoms of hydrogen (below a certain temperature, about 400 000 years after the Big Bang), the electrons and protons were moving like in a plasma. The photons present were scattered all the time, which made the Universe opaque. After the recombination of the two particles, the photons could move freely, ...


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