wiki (https://en.wikipedia.org/wiki/Inflation_(cosmology)) says that immediately after BB there was expansion at speed greater than $c$, what makes this necessary, what would happen if expansion took place at near c?
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6$\begingroup$ No laws are being broken by inflation. There is no theory in physics that states that space has a limit on its expansion rate. $\endgroup$– PraharCommented Sep 14, 2022 at 9:59
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2$\begingroup$ @Prahar, there is no evidence that space is elastic and can expand nor that it can accrue or other, nor that space can drag matter along $\endgroup$– user337596Commented Sep 14, 2022 at 10:02
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10$\begingroup$ @charlie Cosmic inflation is an expansion of the metric of space itself, not an expansion or explosion into space like a supernova. As such, the rate of expansion is not limited by the speed of light. $\endgroup$– gandalf61Commented Sep 14, 2022 at 10:03
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3$\begingroup$ @charlie See expansion of the universe and frame-dragging. $\endgroup$– user76284Commented Sep 14, 2022 at 18:32
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3$\begingroup$ Inflation allows government to... oops, thought I was on a different SO site. $\endgroup$– Wayne ConradCommented Sep 15, 2022 at 16:28
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2 Answers
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There are three commonly-given reasons to believe in cosmic inflation, and they're all given in the Wikipedia article.
- The horizon problem. The universe is homogeneous on large scales when there is no reason it should be.
- The flatness problem. The universe is surprisingly close to flat when there is no reason it should be.
- The monopole problem. Magnetic monopoles are predicted to be copious, but we've not found any. If the theories are right, then they've presumably been diluted (by inflation).
See the Wikipedia article for more details.
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$\begingroup$ It seems like any magnetic monopole breaks conservation of energy since the carrier particle is a photon -- if the monopole is emitting photons (on one pole) without also absorbing them (on the other) isn't that impossible? $\endgroup$– jcollumCommented Sep 15, 2022 at 16:37
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2$\begingroup$ @jcollum Couldn't you claim the same thing about electric monopoles? $\endgroup$– d_bCommented Sep 15, 2022 at 23:32
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$\begingroup$ @charlie en.wikipedia.org/wiki/Flatness_problem#Inflation $\endgroup$– AllureCommented Sep 16, 2022 at 10:06
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Cosmic inflation is an expansion of the metric of space itself, not an expansion or explosion into space like a supernova. As such, the rate of expansion is not limited by the speed of light.
The evidence for the expansion of space is the redshift of light from distant galaxies, and in particular the relation between redshift and distance known as Hubble's law. The fact that redshift only depends on distance an not on direction shows that either space itself is expanding or we are in a unique location from which all galaxies are receding (which is extremely unlikely).
A rapid exponential expansion of space in the first $10^{-32}$ seconds of the universe is required in order to reproduce the observed smoothness of the cosmic microwave background; the finely tuned density of matter and energyin the universe (the flatness problem); and the failure to find evidence of magnetic monopoles in the observable universe.
answered Sep 14, 2022 at 11:37
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4$\begingroup$ Just curious, where does the link between inflation and the absence of magnetic monopoles come from? I'd like to do some reading into this matter. $\endgroup$ Commented Sep 15, 2022 at 7:23
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3$\begingroup$ @ChemiCalChems At extremely high temperatures such as occurred immediately after the Big Bang, magnetic monopole production should have been common. We see no magnetic monopoles today. Inflation explains this by providing a mechanism for dispersing primordial magnetic monopoles. See en.wikipedia.org/wiki/…. $\endgroup$ Commented Sep 15, 2022 at 8:05
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$\begingroup$ @charlie First, expansion of space does not have a fixed speed. The rate of expansion needs to be proportional to distance - otherwise the expansion would have a fixed central point, which we know is incorrect. Secondly the expansion needs to be rapid enough to reproduce the very smooth and flat density pattern that we see in the cosmic microwave background. See en.wikipedia.org/wiki/Flatness_problem for more details. $\endgroup$ Commented Sep 16, 2022 at 9:23