# Tag Info

## Hot answers tagged universe

58

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 ...

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 ...

8

Your description would fit with relative velocities that are constant with time, or decreasing with time, but the evidence suggests they are increasing with time.

2

When people say a specific process "decouples", they mean that it doesn't happen often enough to stay in equilibrium. The reaction $$e^+ + e^- \leftrightarrow \gamma + \gamma$$ can always occur whenever an electron and positron meet, regardless of their energies. But for $T \ll m_e$, the equilibrium number of electrons and positrons is very small, because it'...

2

Good question! I think perhaps the situation you are describing would imply that we are in the center of the universe, i.e. that all of matter is expanding out from some small center, with some distribution in velocities. This would be inconsistent with the dominant viewpoint in science, the Copernican principle, that says that our vantage point is not ...

2

Our clocks cannot measure the duration of inflation directly, since they weren't there at the time. What we can (in principle, in the case of inflation) measure are the signals coming from past events. And yes, if these signals come from near the Big Bang they are massively redshifted and slowed down by the time they get to us. But the times discussed in ...

2

Yes, absolutely. It's well known that the dark matter (DM) fraction varies from galaxy to galaxy, usually as a function of the galaxy (stellar or baryonic) mass. In general, the dark matter fraction is lowest for massive galaxies like our own Milky Way, and increases for both higher-mass galaxies (which are rare) and for lower-mass galaxies (which are common)...

1

In General Relativity you can consider Λ as as related to the intrinsic torsion or curvature of vacuum. In a quantum universe Λ is the zero-point or vacuum ('dark') energy of space-time. Λ$[1/L^2]$ is a bivector, and so the angular acceleration bivector of vacuum is $α=Λ c^2[1/T^2 ]$. To an observer, the radius of the observable Universe is the vector $... 1 The Big Bang time line is seen here: The time is counted from the calculated big bang point, but it is given by our present frame where the clocks are. The inflationary epoch is seen in this plot to be between about$10^{-38}$seconds to$10^{-36}$seconds. In this plot it is seen that a fuzzy region is drawn for the original Big Bang estimate point ... 1 Nope. I think u are mistaken the age of the Universe with the particle horizon which settles the limit of the observable Universe. The$\Lambda$CDM model gives us a value of$d=45 \text{ }\mathrm{by}\$. With time, as the Universe expansion is faster than light for certain distince (Hubble radius), the observable Universe will change, galaxies which are in ...

1

"Equations for the whole universe" were not solved by Einstein. The Universe with her every captivating quirk and je ne sais quoi is impossible to fathom. The only persons who claimed ultimate solution are the fortune tellers and the Wall Street pundits who purportedly can predict tomorrow's SNP 500. However, when it comes to the coarse-grained approximate ...

1

Yes in principle, but no in actuality. Our universe has spacetime curvature, so the spacetime version of the Pythagorean theorem doesn’t hold. Our universe does not have a Minkowski metric. But our universe does not appear to have any measurable spatial curvature, so in only the three spatial dimensions the Pythagorean theorem does hold. However, we can ...

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