The Quest for Cosmic Inflation

Detection of CMB

We now come back to CMB, the original topic of this article. CMB is the key to understanding the properties of our universe. It is the radiation left over when the universe was very hot and dense. This Section presents an account of our current understanding of CMB that emerged from its precision mapping using state of the art telescopes mounted on satellites. These satellite missions have allowed measurement of important cosmological parameters using which scientists have been able to eliminate or set limits on certain classes of inflation models.

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.

Figure 8-1
Figure 4. The minute temperature variations, shown here as shades of blue and purple, are linked to slight density variations in the early universe. These variations are believed to have given rise to clusters of galaxies, as well as vast, empty regions.

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:

Figure 8-2
Figure 5. CMB as observed by WMAP. Notice the higher precision structures in the temperature distribution.
  • The universe is 13.77 \pm 0.5% 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: H_0=69.32 \pm 0.5% km/sec/Mpc.
  • The cosmological parameter was measured to be: \sigma_0 = 1.02 \pm 0.02. 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.

Figure 8-3
Figure 6. CMB as observed by Planck showing temperature fluctuations representing the seeds of stars and galaxies of today. If it is hard to imagine how, the virtual reality tool Exoplanet, can help you to visualize this. The “Galaxy Clusters and CMB” package inside Exoplanet shows several thousand nearby stars and galaxies in our solar neighborhood. Using this package you can zoom further out and see our Milky Way as a part of a sea of other galaxies. Next, you can start zooming in (as if one is travelling backward in time), right to the edge of the universe until the CMB, as observed by Planck, is visible. Recall, CMB is the first light that was able to freely travel through the universe. The positions and distances of each star and galaxy were determined by the Hipparcos satellite, a European mission measuring the parallax of stars (the data was provided by NASA GSFC). The Exoplanet video clip (Exoplanet App version 5.4 – Galaxy Clusters and CMB) shows the animation by starting with our sun and zooming in till CMB is visible, at which point the direction is reversed (zoom out, as if travelling forward in time after CMB is released) until our solar system is eventually visible. Notice, our sun is just an ordinary star residing in an outer arm of an ordinary spiral galaxy, the Milky Way galaxy. In the observable universe the acceptable range for the number of galaxies is between 100 billion and 200 billion. Astronomers have counted the galaxies in a particular region, and multiplied this up to estimate the number for the whole universe.

The table below shows a comparison of the WMAP and Planck measurements of the cosmological parameters.

Table 81: Cosmological parameters
Measured Parameter WMAP Planck
Ordinary matter 4.6% 4.9%
Dark matter 24% 26.8%
Dark energy 71.4% 68.3%
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:

Figure 8-4
Figure 7. Planck detected anomalies in the CMB. One is a large, unexplained cold spot (circled) while the other is an asymmetry in the size of the temperature fluctuations north of the ecliptic compared to the south of the ecliptic, as indicated by the white line. Image: ESA and the Planck Collaboration.
  • 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”.

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