Implications of gravitational waves from the Big Bang?

[laymanB]

My understanding is that gravitational waves have not been detected from the proposed inflationary epoch of the big bang, only from the merger of two super massive objects. Is that correct?

Are GWs predicted from the big bang in models where there is no inflationary epoch?

If GWs are detected from the early universe, what would be the implications for the proposed cosmological models? In other words, would this rule out a lot of contenders?

 

[Chronos]

Generally speaking, primordial gravitational waves [aka CMB B-mode (magnetic) polarization] is a prediction unique to inflationary theory. While you can never say never in science, unequivocal detection of B-mode polarization would leave few hiding places for any models aside from inflation, as discussed here:https://www.scientificamerican.com/article/gravity-waves-cmb-b-mode-polarization/. For additional discussion, see https://arxiv.org/abs/1605.01615, Gravitational waves from inflation.

 

[laymanB]

While you can never say never in science, unequivocal detection of B-mode polarization would leave few hiding places for any models aside from inflation

Thanks. So if the B-mode polarization is found in the CMB, what would the implications for the models be if the tensor-to-scalar ratio r was say, 0.01 versus 0.20 like BICEP2 had originally posited?

 

[laymanB]

Is it a necessary consequence of inflationary models of cosmology that they have to be eternal and/or infinite? I have heard Guth talk this way. Could not a big bang with inflation be a one-time event as opposed to something that is one of a larger set of infinite bubbles/inflations/universes?

 

[bapowell]

There are non-inflationary causes of primordial gravitational waves. See https://arxiv.org/abs/1106.5059. Sorry for the shameless self-promotion but it’s the only reference I have readily at hand.

 

[laymanB]

There are non-inflationary causes of primordial gravitational waves. See https://arxiv.org/abs/1106.5059. Sorry for the shameless self-promotion but it’s the only reference I have readily at hand.

Thanks Brian. The material in your paper is definitely above my knowledge level to evaluate the math and probability arguments, but I see your arguments for B-mode polarization of the CMB coming from sources other than inflation. I'll try to read through it more slowly tomorrow.

 

[bapowell]

This paper might be more useful: https://arxiv.org/pdf/1104.3581.pdf. It’s what motivated me to write mine.

 

[laymanB]

This paper might be more useful: https://arxiv.org/pdf/1104.3581.pdf. It’s what motivated me to write mine.

pg. 9, "Once again, cosmic strings formed in a phase transition during the early Standard Cosmology phase of the evolution of the universe will generate primordial polarization which is statistically equally distributed between E-mode and B-mode [47]."

Several times in the paper the author uses this connection of cosmic strings with standard cosmology. Is that correct?

pg. 9 "More strikingly, if B-mode polarization is found and is shown to be due to gravitational waves, then if the spectrum is slightly blue one would have falsified the inflationary paradigm."

Is there consensus that this slightly blue spectrum result would be a justified reason for falsifying inflation?

 

[laymanB]

Even if the B-mode polarization is found in the CMB, are there any values for the tensor-to-scalar ratio r which would falsify inflation, or would it simply mean that inflation had a shorter or longer duration?

 

[bapowell]

pg. 9, "Once again, cosmic strings formed in a phase transition during the early Standard Cosmology phase of the evolution of the universe will generate primordial polarization which is statistically equally distributed between E-mode and B-mode [47]."

Several times in the paper the author uses this connection of cosmic strings with standard cosmology. Is that correct?

Cosmic strings might have been produced as a result of phase transitions in the early universe (if the particle physics model has the right properties). Brandenberger is just saying that these strings would generate B-mode polarization in the CMB.

Is there consensus that this slightly blue spectrum result would be a justified reason for falsifying inflation?

It would falsify traditional models of inflation, i.e. those for which sensible energy constraints are obeyed (that the energy density drops over the course of inflation.) There are models of so-called "super"-inflation during which the energy density increases, allowing for a blue spectrum, but AFAIK these are not well-motivated physically and suffer from a variety of stability issues.

 

[bapowell]

Even if the B-mode polarization is found in the CMB, are there any values for the tensor-to-scalar ratio r which would falsify inflation, or would it simply mean that inflation had a shorter or longer duration?

Observing a nonzero ts ratio would offer strong support for inflation, whatever the value. Large r simply means that the variation of the inflaton field during inflation was large in Planck units (this tends to cause problems for models of inflation arising from string theory, but that's a potential issue for string theory, not inflation as a cosmological paradigm).

 

[laymanB]

(this tends to cause problems for models of inflation arising from string theory, but that's a potential issue for string theory, not inflation as a cosmological paradigm).

I can imagine that string theorists must have had a roller coaster of emotions as BICEP2 reported the ts ratio at 0.2, and then later discarded the results due to corruption of the data.

 

[bapowell]

I can imagine that string theorists must have had a roller coaster of emotions as BICEP2 reported the ts ratio at 0.2, and then later discarded the results due to corruption of the data.

Yes! Incidentally, there are models of inflation in string theory (monodromy models) that can support large-ish r. I don't know the current state of these models, however.