For example, the variation in density divided by the average density of the universe, δρ/ρ, is of the order of 10 5 for galaxies. However, it is well known that on only slightly smaller scales, up to 120 Mpc, the universe today is very inhomogeneous, consisting of stars, galaxies, and clusters of galaxies. The measured homogeneity and isotropy of the cosmic microwave background radiation temperature (Δ T/ T ∼10 −5) is strong evidence that the observable universe is rather precisely homogeneous and isotropic on the largest scale (1/ H is ∼3000 Mpc). Dykla, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 IV.C Observations and Significance of Large-Scale Structure The observed degree of homogeneity of the temperature of the CMBR thus provides an upper limit on the gravitational wave spectrum Ω gw( f) < 10 −10( H 0/ f) 2( h char( f) < 10 −4( f H/ f) 2) for H 0 < f < 30 H 0.įuture measurements of the polarization of the CMBR with the Microwave Anisotropy Probe (MAP) and the Planck Surveyor satellites will be able to distinguish between scalar perturbations, which are caused by density perturbations, and tensor perturbations, which are caused by gravitational waves, and thus provide a method of directly observing cosmological gravitational waves. This difference in the gravitational potential induces a change in the apparent temperature of the CMBR. The presence of an extremely-low-frequency gravitational wave causes a difference, from point to point in the sky, in the gravitational potential through which the CMBR must travel to reach the earth. However, even if the universe had a perfectly uniform temperature at the time of last scattering, gravitational waves produce observed temperature fluctuations via the Sachs–Wolfe effect. Inflationary models of the very early universe explain this uniformity. Observations of the CMBR show that it is highly uniform, with only very small temperature fluctuations. At that time, known as the time of last scattering, the universe became (essentially) transparent to electromagnetic radiation.
The CMBR was produced in the early universe when the temperature of the universe dropped below ∼3000 K and the plasma of protons and electrons combined to form atomic hydrogen. The measured anisotropy of the Cosmic Microwave Background Radiation (CMBR) places strong constraints on the gravitational wave background from the early universe in the extremely-low-frequency band. Creighton, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 IV.D.1 Microwave Background Radiation Measurements
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