Research Field:
Structures in the Universe

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<b>What is Dark Energy?</b>
Dark energy refers to a mysterious form of energy density which causes an effective repulsive gravitational force, resulting in an accelerated expansion of the universe; by contrast, if the universe were filled only with conventional matter and radiation, the expansion would be slowing down. In the late 1990s, scientists studying the brightness of distant supernovae (exploding stars) were surprised to find evidence for dark energy and also that most of the total energy density (70%) in the universe is from dark energy; the remaining 30% is [mostly dark] matter. Shortly thereafter, additional evidence for dark energy was found in the pattern of temperature fluctuations in the 3 degree cosmic microwave background radiation and in the large-scale distribution of galaxies. Although the dark energy dominates the universe today, theory in fact predicts that its density should be 10¹²⁰ times larger, a puzzle known as the cosmological constant problem. There are currently a number of efforts to probe the nature of the dark energy with greater precision, and in particular determine whether the dark energy density is constant or evolves with cosmic time.

<b>Effects of Dark Energy</b>
To see how weak the effect of dark energy is compared to gravity, it is useful to compute the effect of dark energy on the solar orbit of earth and Pluto.
The acceleration on a planet orbiting the sun is -G*M/R², where G is the gravitational constant, M is the solar mass, and R is the sun-planet distance. The effect of dark energy (DE) is to give a repulsive force; the DE acceleration between the sun and a planet is given by +k*R, where k = 3.7*10^-36/s². Note that with growing distance between two objects, the effect of gravity decreases, while the effect of dark energy increases; for example, doubling the separation between two objects increases the DE-to-gravity acceleration ratio by a factor of eight.
Now let's plug in the numbers. For earth, the acceleration from dark energy is 9.3*10^-23 times that from the sun's gravity. The corresponding change in the earth's orbit, compared to an orbit without dark energy (and with the same orbital velocity), is a tiny 140 nanometers, or about the size of a virus! For Pluto, the distance from the sun is 40 times greater than the earth-sun distance; the corresponding change in Pluto's orbit due to dark energy is 1 micron.

<b>What is a Type Ia Supernova?</b>
A Type Ia supernova is thought to be the explosion of a white dwarf star (consisting of carbon and oxygen) that is accreting mass from a companion star and eventually becomes unstable to thermonuclear runaway when it reaches a critical mass of 1.4 solar masses (called the Chandrasekhar mass, after UChicago physicist Subramanian Chandrasekhar). Since all Type Ia supernovae have about the same mass, they all have about the same explosion energy and therefore similar peak luminosity . . . hence, they are nearly standard candles.

Since the white dwarf contains [almost] no elements lighter than carbon, Type Ia SNe are identified by the lack of hydrogen and helium features in their spectra, and the presence of heavy elements such as silicon, nickel, and iron.

The evolution of a star until explosion is nicely illustrated <a target="_blank" href="http://www.pbs.org/wgbh/nova/universe/super1a.html> here</a>.

Why do we see a Type Ia SN ? With an explosion of such enormous energy, it may seem obvious that it would appear bright in the sky. However, the fact that SNe are visible is due to some rather subtle physics. When a SN explodes, the temperature is billions of degrees, or thousands of times hotter than our sun. The sun's photon spectrum peaks in the optical band (about 1 eV), and the spectrum of a SN explosion peaks in the X-ray band. At the initial explosion, there is far too little light in the optical band to see the SN. If one could "see" X-rays, then you would see an X-ray burst from a Type 1a SN. A future NASA satellite project <a target="_blank" href="http://swift.gsfc.nasa.gov/docs/swift/swiftsc.html">(SWIFT)</a> may be able to see these X-ray bursts. After the SN explosion, it cools extremely rapidly ... in about 20 minutes, the temperature has dropped down to just a few hundred degrees Kelvin. During this rapid cool-down, it passes through the 6000 degree Kelvin temperature of our sun, resulting in strong emission in the optical; unfortunately, this optically bright phase of the cool-down lasts only a few seconds, making it almost impossible to observe.
Following the rapid cool-down of the SN, it seems that the opportunity for strong optical emission has passed. However, recall that in the explosion carbon and oxygen are burnt into heavier elements. Among these heavier elements are radioactive nickel (⁵⁶₂₈Ni) and cobalt (⁵⁶₂₇Co). When these isotopes decay, they emit photons with energies of about 1 MeV (million eV). This 1 MeV energy is still way above the optical, but since these photons are emitted from the inner layers of the SN, they can interact in the outer (cooled) layers. These MeV photons will scatter and be re-emitted many times, a process known as an electromagnetic "shower", and eventually produce the optical light that we see. This conversion of high-energy photons into optical light is very similar to photon detectors used in particle physics; a heavy material such as lead or iron is used to initiate a photon shower, which leads to visible light that is detected with a photomultiplier tube. In the case of a SN, the outer layer (mostly iron) initiates photon showers (where the photons are from radioactive decays), and the visible light is detected with a telescope.

Understanding the source of optical light emission also helps explain why SNe are bright for a few weeks; the half lives of ⁵⁶₂₈Ni and 5627Co are 6 and 78 days respectively. The SN brightness declines rapidly at first due to the decay of nickel; however, the slow component from cobalt decay gives a lower light level that lasts up to a year.

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