Gravitational Waves: Inflation or not?

Nothing gets past you, does it? A scientific paper came out earlier this week, and I took a look at it, sighed, and Jamie asked me, “What?” And I said to her, “When I see bad science, it just makes me a little bit frustrated and sad.” Of course, I had no intention to write about it.

But then Starts With A Bang reader Matt emailed me, and writes the following about this press release that he had seen:

You have two explanations for these gravitational waves now and that much I understand. But they make it sound as if symmetry breaking and inflation are competing theories. They aren’t, right? Do phase transitions influence inflation (it would make sense)? How are those two related? The inflation rate depends on the energy density of the universe (-> scalar fields), right?

And the point is: Even if we attribute the gravitational waves to the process of symmetry breaking, we’d still need to explain the uniformity of the universe because symmetry breaking only explains the origin of the fundamental forces.

So the paper is by Lawrence Krauss, whom I met once back in 2006, when I was giving a talk at Vanderbilt. Lawrence shows up about 40 minutes late (to my one hour talk), makes a scene when he walks in, and demands, “What did I miss?” Feeling indulgent, I gave him a 15 second synopsis of the last 40 minutes, and he goes, with a satisfied smile, “Oh, not much then.” Way to make a good impression on me, Lawrence.

Anyway, his scientific paper doesn’t have anything wrong with it. He basically talks about how a global phase transition can generate gravitational waves, which is 16 year-old news, and those waves might be strong enough to show up in the CMB, just like those from inflation. Is this big news? Come on, anyone can write a paper where you make gravitational waves (the link is to a paper I wrote in 2005).

Here is the important difference, however:

  • Inflation predicts a scale-invariant spectrum.
  • Other mechanisms to make gravitational waves don’t.

A “scale-invariant” spectrum means that energy is evenly distributed in waves of different sizes. Let’s compare the spectrum of inflation (green curve):

to the spectrum in Lawrence’s paper (figure from the paper; he plots things in different units):

and just for fun, let’s throw in the spectrum that my old paper predicts (it’s very different from inflation):

Now, here’s the thing missing from Lawrence’s paper (and admittedly, my paper, too). What is this going to look like in the Cosmic Microwave Background? People have computed it for inflationary models, and know that the shape of the curve should look just like this (the blue curves are for different amplitudes of inflationary models),

so people can go out and try to measure it. Specifically, for those of you who want details, this is looking at the B-mode spectrum of the microwave background, which is one of the things that Planck is designed to measure. What does this new paper predict for their data? Well, they conveniently don’t publish it. Why not? Because it would decidedly be very different from anything resulting from inflation.

Lawrence’s paper talks about something that happens way after the end of inflation, and doesn’t affect the spectrum from inflation or anything related to inflation at all. The paper just gives an extra way to generate gravitational waves of large-enough amplitude that they might show up in the CMB. And they might, if the new physics which he made up is correct. Which, who knows, it might be, and at least we have something new to look for. But this research does nothing to eliminate the need for inflation or change the predictions of inflation, and the press release is indeed wrong for implying that. Thanks, Matt, for forcing me to clear that up.

It’s getting near the end of the week again, so check out this week’s Carnival of Space over at KYsat. Is the KY for the state or the… other thing?


  1. #1 Matt
    April 17, 2008

    Thanks for clearing up the confusion Ethan! :)

  2. #2 ethan
    April 17, 2008

    Bitte schön, Matt!