 What is Dark Energy?
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.
Effects of Dark Energy
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.
How is Dark Energy Detected?
The use of Type Ia supernova as a tool to demonstrate the presence of dark energy was originally reported by two groups: the Supernova Cosmology Project (SCP) and the High-Z SN Team. Below is a simplified explanation of how SNe are used to determine the dark energy content in the universe.
The gravitational effects of the dark energy are too weak to detect in the solar system or even within the galaxy. These effects are observable only on cosmological scales of billions of light years. The most direct method uses Type Ia supernovae because they are nearly standard candles, that is, a class of objects with about the same intrinsic luminosity when they reach their brightest point. The plot below shows the brightness of a Type Ia SNe as a function of the number of days since peak brightness (data provided by SCP).
 The measured redshift (z) gives the cosmological scale factor at the time of explosion. The scale factor "R" is 1 today, and is 1/(1+z) at the time of the SNe explosion. For example, a SNe with redshift z=0.5 corresponds to when the scale factor was 2/3, meaning that the distances between us and the SNe (at explosion time) was 2/3 of the distance today. This redshift is easily measured in SNe, as well as a variety of other cosmological objects such as galaxies, AGNs, and quasars. What make SNe Ia so special is that they also provide a calibrated cosmological time, something that no other object can provide.
To see very roughly how this cosmological time is measured, first consider a simple case with a flat, non-expanding universe; the time of the explosion (relative to today) is simply t = d/c ~ 1/L1/2, where L is the intrinsic SNe luminosity at peak brightness. Since L is the same for each SNe, we have a calibrated cosmological clock. In an expanding universe, the t vs. L relation is more complicated; the key point in this relation is that it also depends on the evolution of matter and dark energy densities. The basic idea is therefore to measure the expansion history of the universe; the scale factor from SNe Ia redshift and the cosmological time from peak-brightness are used to determine the cosmological parameters (matter and dark energy densities) that give the best description of the data.
What is a Type Ia Supernova?
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 (SWIFT) 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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