# Tag Info

## New answers tagged space-expansion

0

If all of physics is reversible like some say, then the the increasing entropy of the Universe is causing us to burn fossil fuels as much as we are increasing the entropy.

3

Okay, let's start with the basics. The Big Bang was not like an explosion in space from which spewed all matter in the universe. The Big Bang was a moment in time. We have this thing called a spacetime metric. I won't bore you with the details, but essentially it is the equation we use to describe all of the geometry in the universe. It includes all the ...

1

The relevant part of the book is the section titled Motion through Spacetime in chapter 2. I'll copy the paragraph, but it's a bit long so feel free to skip over it: Einstein proclaimed that all objects in the universe are always traveling through spacetime at one fixed speed—that of light. This is a strange idea; we are used to the notion that objects ...

0

Everywhere in the observerable universe is affected by gravity and since you can't escape it, that means it is always pulling on you. So every objecct in every place will be moving relative to everything else, so an absolute state of rest is impossible. If you could edit your post to quote the part of the book where he says "stationary", you will probably ...

0

Where does the energy go when light is redshifted? It doesn't go anywhere. Let's say we're motionless with respect to some source which is emitting a stream of photons. We agree that the photons have some energy E=hf. Now let's say I push you such that you're moving away from the source. You now claim that the photons are redshifted, but those photons ...

1

Nobody ever said that different observers have to agree on the energy of a photon (or anything else). The invariant quantity is energy minus momentum (i.e. rest mass), which is equal to zero whether the photon is red or green. (Edited to add: I see now that userLTK already said as much in a comment.)

0

Here is how to see this answer: In the standard model of cosmology, which is given by the FLRW (Friedmann-Lemaitre-Robertson-Walker) solutions of Einstein's field equations, symmetries of isotropy and spatial homogeneity require that such universes be perfect fluid universes. As you know from thermodynamics, perfect fluids have their entropy conserved! So, ...

0

The entropy of a black hole is proportional to its surface area. If the Universe follows the same rule, then as it expands entropy increases, but entropy per volume might be constant, or even decrease. For example, if life continues to increase in its ability to efficiently use Gibbs free energy from Sun photons, fossil fuels, and nuclear sources, it might ...

1

In Minkowski spacetime, the spacetime interval of lightlike movements is zero. That means, from the (hypothetical) point of view of a massless particle such as a photon, it does not even exist one Planck time. At a proper time zero, any wavelength becomes meaningless, even if the physical process is the same that we observe. For the answer you have to take ...

0

Had you asked this question a few billion years ago, I might have told you that space is only expanding this second because it was expanding last second. Back then, the expansion of space was decelerating. The only thing that made it keep expanding was that it had been previously expanding and had not yet slowed to a stop. However, in the cosmologically ...

5

We don't really have a good perspective on what a photon "feels" or, indeed, anything about what its universe would look like. We're massive objects; even the idea of "we must travel at the speed of light because we're massless" makes little sense to us. But we can talk, if you like, about what the world looks like as you travel faster and faster: it's just ...

0

However, if that is the case, than everything would technically pull on everything, right? No. A gravitational field is a place where space is "neither homogeneous nor isotropic". You can See Einstein talking about that here. And the FLRW metric "starts with the assumption of homogeneity and isotropy of space". I'm confident that this is correct ...

0

I describe how to calculate the Hubble parameter in How does the Hubble parameter change with the age of the universe?. You should have a quick read through this as it's relevant to the rest of your question. We know the universe is expanding. We describe its size by a parameter called the scale factor, $a$. The rate of expansion is the rate of change of ...

0

I am attempting answering my question based on one fact and one assumption (actually there are one more fact/ observation - Redshift): The fact is: inertial frame. which, to a good approximation, earth itself (ignore its rotation, irrelevant to our discussion) is an inertial frame. The assumption is: Earth is no special place. Hence the logical ...

0

Measurements of the expansion of the universe fit within the Cosmological Principle. In a nutshell this means that any observer anywhere in the universe sees the same relative motion of distant galaxies. This means that there is no center to the universe.

0

Some galaxies are known to be expanding away from us faster than the speed of light. This is only possible if it is the underlying universe getting bigger. It isn't possible that these galaxies are simply moving faster than the speed of light as your argument suggests as this violates the principles of special relativity.

Top 50 recent answers are included