# Why do we need inflation?

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?

• No laws are being broken by inflation. There is no theory in physics that states that space has a limit on its expansion rate. Sep 14 at 9:59
• @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
– user337596
Sep 14 at 10:02
• @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. Sep 14 at 10:03
• @charlie See expansion of the universe and frame-dragging. Sep 14 at 18:32
• Inflation allows government to... oops, thought I was on a different SO site. Sep 15 at 16:28

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.

• 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? Sep 15 at 16:37
• @jcollum Couldn't you claim the same thing about electric monopoles?
– d_b
Sep 15 at 23:32
• Sep 16 at 10:06

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.

• 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. Sep 15 at 7:23
• @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/…. Sep 15 at 8:05
• @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. Sep 16 at 9:23