It’s not usually put like this, but the discovery of primordial gravitational waves two weeks ago has given us our first direct glimpse of a period before the big bang. [The discovery was later retracted, but I’ll keep this post here to show why theorists at the time found it so exciting.] The term “big bang” is sometimes taken to mean the beginning of the universe, and that’s the impression you get from diagrams such as the one above, which the BICEP2 team showed during the press conference announcing its discovery.

But cosmologists don’t know whether the universe had a beginning. The term “big bang” really refers to the beginning of the universe as we know it—that is, an expanding universe filled with matter that has cooled and coagulated into galaxies. Cosmic inflation, the process the BICEP2 results appear to have vindicated, occurred before the big bang by this definition. The universe during inflation was a deeply alien place, devoid of matter, governed by primeval ur-forces, and thoroughly quantum.

If confirmed by other experiments such as the European Space Agency’s Planck observatory, the waves wash away most competing hypotheses for the prehistorical universe, including the so-called pre-big bang and ekpyrotic scenarios. Scientific American published an article about those alternatives in 2004, written by Gabriele Veneziano, a theoretical physicist who is perhaps best known as the father of string theory. The article included a graph (see below) with predictions—now falsified—for the strength of gravitational waves. (I’d added the BICEP2 result to this graph; note that it’s slightly higher than what was predicted for inflation at the time the article was published.)

Gravitational-wave power spectrum, with BICEP2 result indicated

I caught up with Veneziano in his office at New York University last Friday and, if he was disappointed, he hid it well. Here is an edited version of our conversation.










cosmology particle physics physics

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