Detection of CMB
The discovery of CMB in the 1960’s inspired NASA and the European Space Agency to send a series of satellites into the outer space whose missions were dedicated to cosmology. Their goals were to investigate the CMB radiation of the universe and provide measurements that would help shape scientists’ understanding of the cosmos. The following sections give an overview of the remarkable findings of these missions that have opened up new exciting areas of research in observational and theoretical cosmology.
NASA’s Cosmic Background Explorer (COBE) satellite was developed and built at the Goddard Space Flight Center in Greenbelt, Maryland. COBE, placed into orbit on November 18, 1989, precisely measured and mapped CMB.
COBE’s map of hot and cold spots within this background (Figure 4) is iconic and let scientists have their first glimpse of the baby universe as it appeared only 380,000 years after the Big Bang, showing space speckled with faint spots from which galaxies eventually grew in billions of years. The different colored spots in Figure 4 indicate minute temperature variations. In his book The Universe in a Nut Shell, Stephen Hawking explains how the temperature variations of the hot and cold spots in CMB were responsible for the eventual formation of astronomical objects: “The different colors indicate different temperatures, but the whole range from red to blue is only about a ten thousandth of a degree. Yet this is enough variation between different regions of the early universe for the extra gravitational attraction in the denser regions to stop them expanding eventually, and to cause them to collapse again under their own gravity to form galaxies and stars. So in principle, at least, the COBE map is the blueprint of all the structures in the universe”.
COBE scientists John Mather, at Goddard, and George Smoot, at the University of California, Berkeley, shared the 2006 Nobel Prize in physics for their work. The mission steered cosmologists into a new era of precision measurements.
Launched on June 30, 2001, the Wilkinson Microwave Anisotropy Probe (WMAP) provided the first detailed full-sky map of CMB (Figure 5). After nine years of scanning the sky, WMAP concluded its observations of the CMB. The spacecraft not only gave scientists their best look at this remnant glow, but also established the scientific model that described the history of evolution and structure of the universe.
WMAP measured a host of fundamental cosmological parameters that are crucial to our understanding of the nature of the cosmos. The following summary of findings is an adaptation from WMAP’s website:
- The universe is billion years old.
- The first fine-resolution (0.2°) full-sky map of the microwave sky pattern (tiny fluctuations in the CMB radiation) was obtained.
- First stars ignited 200 million years after the Big Bang.
- Light in WMAP picture is from 379,000 years after the Big Bang.
- Content of the Universe:
- 4.6% ordinary atoms,
- 24% dark matter (matter not made up of atoms),
- Dark energy in the form of a cosmological constant, makes up 71.4% of the universe, causing the expansion rate of the universe to speed up.
- The Hubble constant (expansion rate of the universe) was measured to be: km/sec/Mpc.
- The cosmological parameter was measured to be: . This indicates the universe will expand forever.
- The polarization of the microwave radiation was mapped over the full sky and discovered that the universe was re-ionized earlier than previously believed.
- The amplitude of the variations in the density of the universe on big scales was found to be slightly larger than smaller scales. This, along with other results, supports inflation, the idea is that the universe underwent a dramatic period of expansion, growing by more than a trillion trillion fold in less than a trillionth of a trillionth of a second. Tiny fluctuations were generated during this expansion that eventually grew to form galaxies.
- The distribution of these fluctuations follows a bell curve with the same properties across the sky, and that there are equal numbers of hot and cold spots in the map. The simplest version of the inflation idea predicted these properties and remarkably, WMAP’s precision measurement of the properties of the fluctuations has confirmed these predictions in detail.
CMB was studied in even greater detail by the Planck satellite that was launched in May 2009 by the European Space Agency. Planck data helped to refine some models on the birth of the universe and its subsequent evolution. The age of the universe was measured more accurately but some unexpected results have also surfaced that have sparked excitement among astrophysists.
Planck delivered the most precise all sky image of CMB (Figures 6, 7). The subtle differences in temperature, shown in the image below, are the results of quantum fluctuations in the fabric of the universe when it was just a tiny fraction of a second old. These fluctuations ultimately led to the creation of stars and galaxies.
The table below shows a comparison of the WMAP and Planck measurements of the cosmological parameters.
|Table 8‑1: Cosmological parameters|
|Age of the universe||13.77 billion years||13.82 billion years|
|Hubble constant,||69.32 km/sec/Mpc||67.15 km/sec/Mpc|
|Critical density, Ω0||1.02 ± 0.02||1.02 ± 0.02|
|1 Mpc = 3,260,000 million light years|
First hinted at by WMAP, Planck has also shown several puzzling aspects of the CMB data but with much more clarity:
- Planck showed a giant cold spot in the CMB (Figure 7). This is a particularly surprising result if the universe is truly isotropic and homogenous.
- Presence of an unusual asymmetry in the CMB; the amplitude of the anisotropies are larger on one side of the sky than the other which may imply a preferred direction in space. Even more unusual is that it is lined up with the ecliptic 12. It is not at all understood why the characteristics of CMB should relate to our Solar System.
In summary, the data from COBE, WMAP, and Planck appear to offer striking support for inflation, which has been at the heart of Big Bang cosmology for thirty years. Data was considerably refined in each subsequent satellite mission. These missions supplied the best values known thus far for such critical cosmological parameters as the age of the universe; the curvature of space-time; and when the first atoms, stars, etc. began to form. They have also contributed to our understanding of the fundamental constituents of the universe: dark matter, ordinary matter, and dark energy with their relative proportions. The CMB mapped by these satellites show tiny temperature fluctuations that correspond to regions of slightly different densities at very early times, representing the seeds of all future structures, the stars and galaxies of today. Overall, the information extracted from the CMB map provided an excellent confirmation of the standard model of cosmology. The more precise Planck data allowed one to discriminate between the different models of inflation and also reveal some anomalies in the CMB spectrum that could only have originated in the early universe. These results have far-reaching implications and may lead to refinement of certain class of Inflation models if supported by future observations.
12 Wikipedia defines the ecliptic as “the apparent path of the Sun on the celestial sphere, and is the basis for the ecliptic coordinate system. It also refers to the plane of this path, which is coplanar with both the orbit of the Earth around the Sun and the apparent orbit of the Sun around the Earth. The path of the Sun is not normally noticeable from the Earth’s surface because the Earth rotates, carrying the observer through the cycle of sunrise and sunset, obscuring the apparent motion of the Sun with respect to the stars”.