In the standard model of cosmology,
the early universe underwent a period of fantastic growth. This
inflationary phase, after only a trillionth of a second, concluded
with a violent conversion of energy into hot matter and radiation.
This "reheating" process also resulted in a flood of gravitational
waves. (Interestingly, some cosmologists would identify the "big
bang" with this moment and not the earlier time=0 moment.)
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 (+34-91-497-4896, firstname.lastname@example.org) 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.
For comparable results see the paper by Easther and Lim in the Journal of Astroparticle Physics, JCAP04(2006)010.
Physical Review Letters, upcoming article
Contact Juan Garcia-Bellido
Universidad Autonoma de Madrid
Also see The UAM Lattice Field Theory Group