Gravitational Wave Background

Let's compare this gravitational wave background (GWB) with the more familiar cosmic microwave background (CMB). The GWB dates from the trillionth-of-a-second mark, while the CMB sets in around 380,000 years later when the first atoms formed. The CMB represents a single splash of photons which were (at that early time) in equilibrium with the surrounding atoms-in-the-making; the microwaves we now see in the sky were (before being redshifted to lower frequencies owing to the universe's expansion) ultraviolet waves and were suddenly freed to travel unimpeded through space. They are now observed to be mostly at a uniform temperature of about 3 degrees Kelvin, but the overall map of the microwave sky does bear the faint imprint of matter inhomogeneities (lumps) existing even then.

What, by contrast, does the GWB represent? It stems from three different production processes at work in the inflationary era: waves stemming from the inflationary expansion of space itself; waves from the collision of bubble-like clumps of new matter at reheating after inflation; and waves from the turbulent fluid mixing of the early pools of matter and radiation, before equilibrium among them (known as thermalization) had been achieved. The gravity waves would never have been in equilibrium with the matter (since gravity is such a weak force there wouldn't be time to mingle adequately); consequently the GWB will not appear to a viewer now to be at a single overall temperature.

A new paper by Juan Garcia-Bellido and Daniel Figueroa (Universidad Autonoma de Madrid) explain how these separate processes could be detected and differentiated in modern detectors set up to see gravity waves, such as LIGO, LISA, or BBO (Big Bang Observer). First, the GWB would be redshifted, like the CMB. But because of the GWB's earlier provenance, the reshifting would be even more dramatic: the energy (and frequency) of the waves would be downshifted by 24 orders of magnitude. Second, the GWB waves would be distinct from gravity waves from point sources (such as the collision of two black holes) since such an encounter would release waves with a sharper spectral signal. By contrast the GWB from reheating after inflation would have a much broader spectrum, centered around 1 hertz to 1 gigahertz depending on the scale of inflation.

Garcia-Bellido suggests that if a detector like the proposed BBO could disentangle the separate signals of the end-of-inflation GWB, then such a signal could be used as a probe of inflation and could help explore some fundamental issues as matter-antimatter asymmetry, the production of topological defects like cosmic strings, primoridal magnetic fields, and possibly superheavy dark matter.