I spent a few days last week to prepare for presentation today at our own journal club ACϟJC. The presentation was mainly about addressing the question that a cosmologist colleague asked me right after the big discovery of BICEP2. I learnt a lot during the preparation for this talk, mainly about early universe, low-frequency gravitational-waves detection, and pulsar timing arrays. The question was the following: is it possible to detect gravitational waves from inflation?
Before we address the question let’s ask ourselves why one should ask this question and why this question is important? Well, this is because gravitational waves are our only hope (that we are aware of so far, at least) to look at very early universe (and by “very” early universe I mean before CMB phase in which the universe is so dense and hot such that no photon can scape, simply because of too much scattering!_see this video for details_). Gravitational waves produced by quantum fluctuations just after the big bang can travel from very early universe all the way to our current detectors without any major interaction! Primordial gravitational waves can provide lots of information about the very early universe for the first time that we currently know basically nothing about it… A baby picture of the universe, in another words! It’s as much cool and exciting as finding a picture of you when you were just born… So that’s why we want to know if it is possible to detect gravitational waves from inflation or not.
Notice that our ability of detection does not only depend on the properties of the waves but also depends on the sensitivity of our detectors too. So, the more interesting question could be the following: Will we be able to detect inflationary gravitational waves by the current proposed detectors (like DECIGO and BBO)? The short answer is yes, it is possible.
The more technical answer is that it will also depend a bit on the details of our cosmological model too. More specifically, it depends on the value of reheating temperature after the inflation which we are not certain about its value yet. With the current proposes very-low-frequency space-based detectors such as DECIGO and BBO and extremely-low-frequency pulsar timing detectors (such as IPTA and SKA), under some certain cosmological conditions (e.g. having no preheating and reheating temperature greater than some certain value), it is totally possible to detect gravitational waves from inflation by these detectors (see above figures to get some quantitative sense).