Dark Energy – A Factor on Universe Expansion
Dark Energy â A Factor on Universe Expansion
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Home Page > Education > Science > Dark Energy â A Factor on Universe Expansion
Dark Energy â A Factor on Universe Expansion
Posted: Nov 26, 2010 |Comments: 0
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Dark energy is a dilute component that has been proposed to explain the observed recent acceleration of the universe’s expansion, which can be accounted for a wide range of observations (1). Dark energy- the mysterious stuff that is causing the universe’s expansion to accelerate â could be illuminated by another dark enigma: the black hole. The dark energyis a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the Universe (2). Dark energy is the most popular way to explain recent observations and experiments that the Universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 74% of the total mass-energy of the Universe(3).Â
 Einstein’s static Universe would actually be unstable because local non-homogeneities would ultimately lead to either the runaway expansion or contraction of the Universe. The equilibrium is unstable: if the Universe expands slightly, then the expansion releases vacuum energy, which causes yet more expansion. Likewise, a Universe which contracts slightly will continue contracting. These sorts of disturbances are inevitable, due to the uneven distribution of matter throughout the universe. More importantly, observations made by Edwin Hubble showed that the universe appears to be expanding and not static at all (4).
The discovery in 1998 that the Universeis actually speeding up its expansion was a total shock to astronomers. It just seems so counter-intuitive, so against common sense. But the evidence has become convincing. The evidence came from studying distant Type Ia Supernovae. This type of Supernova results from having a White Dwarf Starin a binary system. Mattertransfers from explosion. Because all White Dwarfs achieve the same mass before exploding, they all achieve the same luminosityand can be used by astronomers as “standard candles.” Thus by observing their apparent brightness, astronomers can determine their distance using the 1/r2 law. By knowing the distance to these Supernovae, astronomers know how long ago they occurred. In addition, the lightfrom the Supernova has been Red-shifted by the expansion of the Universe. By measuring this Red-shiftfrom the spectrumof the Supernova, astronomers can determine how much the Universe has expanded since the explosion. By studying many Supernovae at different distances, astronomers can piece together a history of the expansion of the Universe. In the 1990’s two teams of astronomers, the Supernova CosmologyProject (Lawrence Berkeley National Laboratory) and the High-ZSupernova Search (international), were looking for distant Type Ia Supernovae in order to measure the expansion rate of the Universe with time. They expected that the expansion would be slowing, which would be indicated by the Supernovae being brighter than their Redshifts would indicate. Instead, they found the supernovae to be fainter than expected. Hence, the expansion of the Universe was accelerating!(5)
Super-massive Black holes are thought to exist at the core of most Galaxies. When two Galaxies collide and merge, these Black holes will go into orbit around each other, spiral inwards and eventually collide. According to Einstein’s theory of general relativity, two orbiting Black holes will generate powerful gravitational waves, ripples in the fabric of space-time. NASA and European Space Agency (ESA) planed a joint mission called LISA to detect these waves. The distances and redshifts of orbiting Black holes give us the history of expansion of the Universe (6).
In addition, measurements of the cosmic microwavebackground indicate that the Universe has a flat geometry on large scales. Because there is not enough matter in the Universe — either ordinary or dark matter– to produce this flatness, the difference must be attributed to a “Dark energy”. This same dark energy causes the acceleration of the expansion of the Universe. In addition, the effect of dark energy seems to vary, with the expansion of the Universe slowing down and speeding up over different times. Astronomers know dark matter is there by its gravitationaleffect on the matter that we see, and there are ideas about the kinds of particles it must be made of. By contrast, dark energy remains a complete mystery. The name “Dark energy” refers to the fact that some kind of “stuff” must fill the vast reaches of mostly empty space in the Universe in order to be able to make space accelerate in its expansion. In this sense, it is a “field” just like an electric field or a magnetic field, both of which are produced by electromagnetic energy. But this analogy can only be taken so far, because we can readily observe electromagnetic energy via the particle that carries it, the photon(5).
In the context of dark energy, the cosmological constant is a reservoir which stores energy. Its energy scales as the Universe expands. Applied to the supernova data, it would distinguish effects due to the matter in the Universe from those due to the dark energy. Unfortunately, the amount of this stored energy required is far more than observed and would result in very rapid acceleration — so much so that the stars and Galaxieswould not form. Physicists have suggested a new type of matter, “quintessence,” which would fill the Universe like a fluid which has a negative gravitational mass. However, new constraints imposed on cosmological parameters by Hubble Space Telescope data rule out at least simple models of quintessence. (5)
In quintessence models of dark energy, the observed acceleration of the scale factor is caused by the potential energy of a dynamical field, referred to as quintessence field. Quintessence differs from the cosmological constant in that it can vary in space and time. In order for it not to clump and form structure like matter, the field must be very light so that it has a large Compton wavelength. No evidence of quintessence is yet available, but it has not been ruled out either. It generally predicts a slightly slower acceleration of the expansion of the universe than the cosmological constant. Some scientists think that the best evidence for quintessence would come from violations of Einstein’s equivalence principle and variation of the fundamental constants in space or time. Scalar fields are predicted by the standard model and string theory, but an analogous problem to the cosmological constant problem (or the problem of constructing models of cosmic inflation) occurs: renormalization theory predicts that scalar fields should acquire large masses (4).
Other possibilities being explored are topological defects, time-varying forms of dark energy, or a dark energy that does not scale uniformly with the expansion of the Universe (5). The pattern of gravitational waves from colliding black holes can tell us how far they are from us. By studying the distance and red shifts of Black hole pairs at different eras of cosmic time, it is possible to calculate the expansion history of the universe(6).
The recent acceleration of the Universe, whether caused by dark energy or a manifestation of a modification of gravity on scales a hundred times larger than the solar system, presages new physics. Experiments in the next decade promise even greater support about the relation of dark energy with expansion history of the Universe.
References
1.        Spergel, D.N. et al. (2007) Astrophys.J. Suppl. Ser. 170, 377-408
2.        P. J. E. Peebles and Bharat Ratra (2003). “The cosmological constant and dark
           energy” (http://www.arxiv.org/abs/astro-ph/0207347) (subscription required).        Reviews of Modern Physics 75:
3.        Hinshaw, Gary F. (April 30th, 2008).            (http://lambda.gsfc.nasa.gov/product/map/dr3/params/lcdm_sz_lens_wmap5.cfm) . NASA.
4.        http://en.wikipedia.org/wiki/Dark_energy
5.        http://imagine.gsfc.nasa.gov/docs/science/mysteries_l1/dark_energy.html  Â
6.        Stephen Battersby. NewScientist, 5th September, 2009.
.
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Dr. Manisha Majumdar (De) –
About the Author:
I was born in Kolkata, Qualified Ph.D on 1989 from Calcutta University (Spl. Endocrinology), acquired research experience of more than 22 years with publications of around 29 papers in various national / international journals , acquired teaching experience of more than 15 years, acquired experience of writing biology text book under ISC course which is currently under Cambridge press (Kolkata) for publication. Awarded Sangit Prabhakar and Prayag Sangit Samiti in Indian Classical music. Awarded certificate of appreciation â Celebrations of the centenary of Ramkrishna Mission & of Swamy Vivekananda’s historic return from west in 1897 as well as of 66th foundation day of the pratisthan. Ramakrishna Mission Seva Pratisthan (Kol-India), expressed its participation of my contribution to the success of the seminar in 1997.Â
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dark energy, supernova, black hole, universe
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I was born in Kolkata, Qualified Ph.D on 1989 from Calcutta University (Spl. Endocrinology), acquired research experience of more than 22 years with publications of around 29 papers in various national / international journals , acquired teaching experience of more than 15 years, acquired experience of writing biology text book under ISC course which is currently under Cambridge press (Kolkata) for publication. Awarded Sangit Prabhakar and Prayag Sangit Samiti in Indian Classical music. Awarded certificate of appreciation – Celebrations of the centenary of Ramkrishna Mission & of Swamy Vivekananda’s historic return from west in 1897 as well as of 66th foundation day of the pratisthan. Ramakrishna Mission Seva Pratisthan (Kol-India), expressed its participation of my contribution to the success of the seminar in 1997.Â
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