# Tag Info

13

It is unlikely that we can detect gravitational waves from the Big Bang with current technology. Due to universal expansion, such waves would have very large wavelengths. We would need interferometers that are thousands, perhaps millions, of kilometers long to detect them. The LIGO observatory simply would not be sufficient. To put things into perspective, ...

7

There is much less understood about what kind of "cosmic background" may exist for gravitational waves than for electromagnetic waves. The CMB comes from a specific epoch -- when the universe became transparent to light, and the electromagnetic field, previously in equilibrium with matter, was "released" as a self-contained thermal ...

4

What typical wavelength would a gravitational wave background have? The answer is: all frequencies up to some cut-off scale, and above that not much. So there are long wavelengths and short, but not very short. The details are not known but some models suggest a scale-invariant spectrum, which means you have equal amounts of power in each fractional ...

3

We should be a bit precise with terminology here. A perfect fluid is a fluid with stress tensor $T^{\mu \nu} = p g^{\mu \nu} + (\rho + p) u^\mu u^\nu$ (using -+++ signature) where $u^\mu$ is the fluid velocity vector satisfying $u_\mu u^\mu = -1$. Further, $\rho$ and $p$ are not independent; they are related by an equation of state. In full generality this ...

1

The New Scientist article and the paper are from 2013. That was before they did new measurements of the top quark mass etc. https://arxiv.org/pdf/1707.08124.pdf is the most recent AFAIK, it's had four updates, most recent in 2018. https://arxiv.org/abs/1707.08124 The latest version of the Wikipedia article removes those cites - probably because a later ...

1

The universe would be infinitely large now if it started out infinitely large. If the universe is infinitely large it can still expand, in the sense that the distances between the galaxies can get larger over time. But we have no way to know whether it is infinitely large. It could be finite. I think it is possible that when the concept of "infinity&...

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