Category Archives: Dark Energy

Looking for Dark Energy in the Lyman Alpha Forest

Baryon acoustic oscillations (BAO) are the acoustic (sound) waves that occur in the very early universe due to very small density inhomogeneities in the nearly uniform fluid. These primordial acoustic oscillations have left an imprint on the way in which galaxies are spatially distributed. The characteristic scale length for these oscillations is around 500 million light years (in the frame of the present-day universe). A spatial correlation function is used to measure the degree to which galaxies and clumps of matter in general, including dark matter, are separated from one another. The characteristic length scale serves as a standard ruler for very large-scale clustering, and is seen as a distinctive break (change in slope) in the power spectrum of the degree of spatial correlation vs. distance.

An international consortium of astronomers representing 29 institutions have submitted a paper last month to the journal Astronomy and Astrophysics; the current version can be found here (http://arxiv.org/abs/1211.2616). They used a clever technique of detecting clouds of neutral hydrogen along the line of sight to a large number of quasars with high redshifts. Thus the matter clumps in this case are neutral hydrogen clouds or neutral hydrogen within intervening galaxies or proto-galaxies. These absorb light from the quasar and produce absorption lines in the spectrum at discrete locations corresponding to various redshifts. The authors are detecting a characteristic transition known as the Lyman alpha line, which is found well into the ultraviolet at 121.6 nanometers (for zero redshift).

For this work over 48,000 high-redshift quasars, with a mean redshift of 2.3, were taken from the 3rd Sloan Digital Sky Survey. A quasar may have many hydrogen clouds intervening along the line of sight from the Earth to the quasar. These clouds will be seen at different redshifts (less than the quasar redshift) reflecting their position along the line of sight. This is referred to as the Lyman alpha “forest”. This study is the first application of Lyman alpha forest measurement to the detection of the BAO feature. At the average red shift of 2.3, the wavelength of Lyman alpha radiation is shifted to 401.3 nm [calculated as (1+2.3)*121.6 nm], in the violet portion of the visible spectrum. The study incorporated redshifts from the absorbing clouds in the range of 1.96 to 3.38; these are found in front of (at lower redshift than) quasars with redshifts ranging from 2.1 to 3.5.

Speedup or slowdown versus age of universe. The Big Bang is on the left, 13.7 billion years ago.

The authors’ measurement of the expansion rate of the universe is shown as the red dot in this figure. The white line through the various data points is the rate of expansion of the universe expected versus time for the standard Lambda-Cold Dark Matter cosmological model. The expansion rate at early times was lessening due to gravity from matter (ordinary and dark), but it is now increasing, since dark energy has come to dominate during the last 5 billion years or so. The red data point is clearly on the slowing down portion of the curve. Image credit: http://sdss3.wordpress.com/2012/11/13/boss-detects-baryon-acoustic-oscillations-in-the-lyman-alpha-forest-at-z-of-2-3/  

The BAO feature has been measured a number of times, using galaxy spatial distributions, but always at lower redshifts, that is at more recent times. This is not only the first Lyman alpha-based measurement, but the first measurement made at a high redshift for which the universe was still slowing down, i.e. the expansion was decelerating. At the redshift of 2.3, when the universe was only about 3 billion years old, the gravitational effect of dark plus ordinary matter was stronger than the repulsive effect of dark energy. It is only more recently, after the universe become about 9 billion years old (some 5 billion years ago), and corresponding to redshifts less than about z = 0.8, that dark energy began to dominate and cause an acceleration in the overall expansion of the universe.

Since this observation shows a significantly higher rate of expansion than occurred at the minimum around 5 billion years ago, it is further evidence that dark energy in some form is real. As the authors state in their paper: “Combined with CMB constraints, we deduce the expansion rate at z = 2.3 and demonstrate directly the sequence of deceleration and acceleration expected in dark-energy dominated cosmologies.” This is an exciting result, providing additional confirmation that dark energy represents around three-quarters of the present-day energy balance of the universe.


Dark Energy Survey First Light!

Last month the Dark Energy Survey project achieved first light from its remote location in Chile’s Atacama Desert. The term first light is used by astronomers to refer to the first observation by a new instrument.

And what an instrument this is! It is in fact the world’s most powerful digital camera. This Dark Energy Camera, or DECam, is a 570 Megapixel optical survey camera with a very wide field of view. The field of view is over 2 degrees, which is rather unusual in optical astronomy. And the camera requires special CCDs that are sensitive in the red and infrared parts of the spectrum. This is because distant galaxies have their light shifted toward the red and the infrared by the cosmological expansion. If the galaxy redshift is one,  the light travels for about 8 billion years and the wavelength of light that the DECam detects is doubled, relative to what it was when it was originally emitted.

Dark Energy Camera

Image: DECam, near center of image, is deployed at the focus of the 4-meter Victor M. Blanco optical telescope in Chile (Credit: Dark Energy Survey Collaboration)

The DECam has been deployed to further our understanding of dark energy through not just one experimental method, but in fact four different methods. That’s how you solve tough problems – by attacking them on multiple fronts.

It’s taken 8 years to get to this point, and there have been some delays, as normal for large projects. But now this new instrument is mounted at the focal plane of the existing 4-meter telescope of the National Science Foundation’s Cerro Tololo Inter-American observatory in Chile. It will begin its program of planned measurements of several hundred million galaxies starting in December after several weeks of testing and calibration. Each image from the camera-telescope combination can capture up to 100,000 galaxies out to distances of up to 8 billion light years. This is over halfway back to the origin of the universe almost 14 billion years ago.

In a previous blog entry I talked about the DES and the 4 methods in some detail. In brief they are based on observations of:

  1. Type 1a supernova (the method used to first detect dark energy)
  2. Very large scale spatial correlations of galaxies separated by 500 million light-years (this experiment is known as Baryon Acoustic Oscillations since the galaxy separations reflect the imprint of sound waves in the very early universe, prior to galaxy formation)
  3. The number of clusters of galaxies as a function of redshift (age of the universe)
  4. Gravitational lensing, i.e. distortion of background images by gravitational effects of foreground clusters in accordance with general relativity

NGC 1365

Image: NGC 1365, a barred spiral galaxy located in the Fornax cluster located 60 million light years from Earth (Credit: Dark Energy Survey Collaboration)

What does the Dark Energy Survey team, which has over 120 members from over 20 countries, hope to learn about dark energy? We already have a good handle on its magnitude, at around 73% presently of the universe’s total mass-energy density.

The big issue is does it behave as a cosmological constant or as something more complex? In other words, how does the dark energy vary over time and is there possibly some spatial variation as well? And what is its equation of state, or relationship between its pressure and density?

With a cosmological constant explanation the relationship is Pressure = – Energy_density, a negative pressure, which is necessary in any model of the dark energy, in order for it to drive the accelerated expansion seen for the universe. Current observations from other experiments, especially those measuring the cosmic microwave background, support an equation of state parameter within around 5% of the value -1, as represented in the equation in the previous sentence. This is consistent with the interpretation as a pressure resulting from the vacuum. Dark energy appears also to have a constant or nearly constant density per unit volume of space. It is unlike ordinary matter and dark matter, that both drop in mass density (and thus energy density) as the volume of the universe grows. Thus dark energy becomes ever more dominant over dark matter and ordinary matter as the universe continues to expand.

We can’t wait to see the first publication of results from research into the nature of dark energy using the DECam.

References:

http://www.noao.edu/news/2012/pr1204.php – Press release from National Optical Astronomical Observatory on DECam first light

www.darkenergysurvey.org

http://www.ctio.noao.edu/noao/ – Cerro Tololo Inter-American Observatory page

http://lambda.gsfc.nasa.gov/product/map/dr4/pub_papers/sevenyear/basic_results/wmap_7yr_basic_results.pdf – WMAP 7 year results on cosmic microwave background

https://darkmatterdarkenergy.com/2011/03/08/dark-energy-survey/


Future of Our Runaway Universe (the next Trillion Years)

Future for our Sun: Ultraviolet image of the planetary nebula NGC 7293 also known as the Helix Nebula. It is the nearest example of what happens to a star, like our own Sun, as it approaches the end of its life when it runs out of fuel, expels gas outward and evolves into a much hotter, smaller and denser white dwarf star. Image Credit: NASA/JPL-Caltech/SSC

In the future, the average density of matter in the universe (both ordinary matter and dark matter) will continue to drop in proportion to the increasing spatial volume as the universe expands ever more rapidly. The dark energy density, however, behaves differently. Dark energy is an irreducible property of even empty space, so as new space is created, the dark energy density remains the same; it is believed to not only take the same value in all portions of space at a given time, but to also have had the same value (per unit volume) for many billions of years.

Since around 5 billion years ago, when the universe was 9 billion years old, the dark energy has dominated over both types of matter (ordinary and dark) and this dominance is only increasing with the universe’s continued expansion. Today it is 73% of the total mass-energy density and it will approach close to 100% in the future. The assumption is made that the cosmological constant or dark energy term that we measure today remains constant into the future. However it cannot be ruled out that it is changing very slowly or might change suddenly at some future date.

In the cosmological constant case, the scale factor for the size of the universe grows exponentially with time. This is known as the de Sitter solution to the equations of general relativity, and it indicates that the expansion of the universe is accelerating into a runaway condition. There is a single parameter, a timescale. Cosmological measurements indicate that the value is such that the size of the universe for each spatial dimension will double and redouble every 11 billion years (the volume will thus grow by 8 times each 11 billion years).

When the universe is 25 billion years old (now it’s 14 billion years old), distant galaxies will be about twice as far away as today (and 4 times fainter). Well before that time we’ll need to evacuate the Earth as the Sun will go into its red giant phase some 5 billion years from now, followed by a white dwarf phase – as shown in the image of the Helix planetary nebula above. When the universe is around 124 billion years old, distant galaxies on average will be 1000 times farther away from us than now. And after 234 billion years they will be an incredible million times farther away than now!

Year                                    Relative Distance                        Relative Brightness

14 billion (Now)                        1                                                1

25 billion                                    2                                                1/4

124 billion                                  1000                                         one-millionth

234 billion                                  1,000,000                               one-trillionth

The distant galaxies that we detect with the Hubble telescope and large Earth-bound telescopes will become invisible since their apparent luminosity will drop as the square of the increasing distance. For example at the time of 124 billion years, they will be 1 million times fainter (1000 squared). At the time of 234 billion years they will be a trillion times fainter (one million squared). Actually it will be worse than this since their light will be redshifted (stretched out by the cosmological expansion) by the same relative distance factor, so light emitted in the visible will be detected in the millimeter radio region when the universe is 100+ billion years old. This is without considering the evolution in their stellar populations, but only their lower mass, fainter stars will survive, further aggravating the situation.

Galaxies themselves are not changing very much in their size or in internal density, rather it is the spacing between galaxies that is on average growing rapidly. Galaxy groups and clusters that are today gravitationally bound will remain bound. Our home, the Milky Way galaxy, and its large neighbor the Andromeda galaxy, will stay together since they are gravitationally bound, and they may very well merge in several billion years due to tidal effects. All of the 40 or so galaxies and dwarf galaxies in our gravitationally bound Local Group may coalesce after 1 trillion years have passed.

Our light cone horizon, which determines which galaxies are even theoretically visible to us, is shrinking in relative terms. Sufficiently distant galaxies are already receding faster than the speed of light from our vantage point and are entirely hidden from us; if the inflationary model is correct as seems to be the case, the universe is immensely larger than what we are able to detect. This is possible and indeed happening because there are no constraints in special relativity or general relativity on the expansion rate of space itself; only the objects within space are constrained to moving at less than the speed of light relative to their local frames of reference.

An intelligent society in the very distant future, possibly our descendants who have moved to a planet in orbit around another star, would observe only one galaxy, namely their own. This would be a larger galaxy formed from the Milky Way and other members of the Local Group. All other galaxies would no longer be visible, first they would become too distant and too faint, and then they would be entirely beyond our light horizon. These descendants or other observers would believe their galaxy to be the only one in the universe, unless they had access to (and a willingness to believe in) very ancient research publications.

We are fortunate to live in this epoch – despite dark matter, dark energy, and dark gravity, the universe is young, and we are immersed in light.

References:

http://spiff.rit.edu/classes/phys240/lectures/future/future.html

The Five Ages of the Universe, Fred Adams and Greg Laughlin, Simon and Schuster, 1999

The Runaway Universe, Donald Goldsmith, Perseus Books, 2000

Dark Matter, Dark Energy, Dark Gravity, Stephen Perrenod, 2011, https://darkmatterdarkenergy.wordpress.com/where-to-find/


2011 Nobel Prize for Dark Energy Discovery

Measurements of Dark Energy and Matter content of Universe

Dark Energy and Matter content of Universe: The intersection of the supernova (SNe), cosmic microwave background (CMB) and baryon acoustic oscillation (BAO) ellipses indicate a topologically flat universe composed 74% of dark energy (y-axis) and 26% of dark matter plus normal matter (x-axis).

The 2011 Nobel Prize in Physics, the most prestigious award given in the physics field, was announced on October 4. The winners are astronomers and astrophysicists who produced the first clear evidence of an accelerating universe. Not only is our universe as a whole expanding rapidly, it is in fact speeding up! It is not often that astronomers win the Nobel Prize since there is not a separate award for their discipline. The discovery of the acceleration in the universe’s expansion was made more or less simultaneously by two competing teams of astronomers at the end of the 20th century, in 1998, so the leaders of both teams share this Nobel Prize.

The new Nobel laureates, Drs. Saul Perlmutter, Adam Riess, and Brian Schmidt, were the leaders of the two  teams studying distant supernovae, in remote galaxies, as cosmological indicators. Cosmology is the study of the properties of the universe on the largest scales of space and time. Supernovae are exploding stars at the ends of their lives. They only occur about once each fifty to one hundred years or so in a given galaxy, thus one must study a very large number of galaxies in an automated fashion to find a sufficient number to be useful. The two teams introduced new automated search techniques to find enough supernovae and achieve their results.

During a supernova explosion, driven by rapid nuclear fusion of heavy elements, the supernova can temporarily become as bright as the entire galaxy in which it resides. The astrophysicists studied a particular type of supernova known as Type Ia. These are due to white dwarf stellar remnants exceeding a critical mass. Typically these white dwarfs would be found in binary stellar systems with another, more normal, star as a companion. If a white dwarf grabs enough material from the companion via gravitational tidal effects, that matter can “push it over the edge” and cause it to go supernova. Since all occurrences of this type of supernova event have the same mass for the exploding star (about 1.4 times the Sun’s mass), the resultant supernova has a consistent brightness or luminosity from one event to the next.

This makes them very useful as so-called standard candles. We know the absolute brightness, which we can calibrate for this class of supernova, and thus we can calculate the distance (called the luminosity distance) by comparing the observed brightness to the absolute. An alternative measure of the distance can be obtained by measuring the redshift of the companion galaxy. The redshift is due to the overall expansion of the universe, and thus the light from galaxies when it reaches us is stretched out to longer, or “redder” wavelengths. The amount of the shift provides what we call the redshift distance.

Comparing these two different distance techniques provides a cosmological test of the overall properties of the universe: the expansion rate, the shape or topology, and whether the expansion is slowing down, as was expected, or not. The big surprise is that the expansion from the original Big Bang has stopped slowing down due to gravity and has instead been accelerating in recent years! The Nobel winners did not expect such a result, thought they had made errors in their analyses and checked and rechecked. The acceleration did not go away. And when they compared the results between the two teams, they realized they had confirmed each others’ profound discovery of the reality of a dark energy driven acceleration.

The acceleration result is now well founded since it can be seen in the high spatial resolution measurements of the cosmic microwave background radiation as well. This is the radiation left over from the Big Bang event associated with the origin of our universe.

The acceleration is now increasingly important, dominating during the past 5 billion years of the 14 billion year history of the universe. Coincidentally, this is about how long our Earth and Sun have been in existence. The acceleration has to overcome the self-gravitational attraction of all the matter of the universe upon itself, and is believed to be due to a nonzero energy field known as dark energy that pervades all of space. As the universe expands to create more volume, more dark energy is also created! Empty space is not empty, due to the underlying quantum physics realities. The details, and why dark energy has the observed strength, are not yet understood.

Amazingly, Einstein had added a cosmological constant term, which acts as a dark energy, to his equations of General Relativity even before the Big Bang itself was discovered. But he later dropped the term and called it his worst blunder, after the expansion of the universe was first demonstrated by Edwin Hubble over 80 years ago. It turns out Einstein was in fact right; his simple term explains the observed data and the Perlmutter, Riess, and Schmidt measurements indicate that ¾ of the mass-energy content of the universe is found in dark energy, with only ¼ in matter.

Our universe is slated to expand in an exponential fashion for trillions of years and more, unless some other physics that we don’t yet understand kicks in. This is rather like the ever-increasing pace of modern technology and modern life and the continuing inflation of prices.

We honor the achievements of Drs. Perlmutter, Riess, and Schmidt and of their research teams in increasing our understanding of our universe and its underlying physics. Interestingly, only a few weeks ago, a very important supernova in the nearby M101 galaxy was discovered, and it is also a Type 1a. Because it is so close, only 25 million light years away, it is yielding a lot of high quality data. Perhaps this celestial fireworks display was a harbinger of their Nobel Prize?

References:

http://www.nytimes.com/aponline/2011/10/04/science/AP-EU-SCI-Nobel-Physics.html?_r=2&hp

http://www.nobelprize.org/nobel_prizes/physics/laureates/2011/press.html

http://www.nobelprize.org/mediaplayer/index.php?id=1633 (Telephone interview with Adam Reiss)

http://supernova.lbl.gov/ (Supernova Cosmology Project)

https://darkmatterdarkenergy.wordpress.com/2011/08/31/m101-supernova-and-the-cosmic-distance-ladder/

https://darkmatterdarkenergy.wordpress.com/2011/07/04/dark-energy-drives-a-runaway-universe/

 


M101 Supernova and the Cosmic Distance Ladder

Last week, on August 24, there was a very fortuitous and major discovery by UC Berkeley and Lawrence Berkeley National Lab astronomers of a nearby Type 1a supernova, named PTF 11kly, in the nearby Pinwheel Galaxy. This galaxy in Ursa Major is also known as M101 (the 101st member of the Messier catalog). Type 1a supernovae are key to measuring the cosmological distance scale since they act as “standard candles”, that is, they all have more or less the same absolute brightness. Dark energy was first discovered through Type 1a supernovae measurements. These supernovae are due to certain white dwarf runaway thermonuclear explosions.

Supernova in M101

Supernova in M101 (Credit: Lawrence Berkeley National Laboratory, Palomar Transient Factory team)

Three photos on 3 successive nights, with the supernova not detectable on 22 August (left image), detectable (pointed to by green arrow) on 23 August (middle image) and brighter on 24 August (right image).

A white dwarf is the evolutionary end state for most stars, including our Sun eventually, after it exhausts the hydrogen and helium in its core via thermonuclear fusion. Some of the star’s outer envelope is ejected during a nova phase but the remaining portion of the star collapses dramatically, until it is only about the size of the Earth. This is due to the lack of pressure support that previously was generated by nuclear fusion at high temperatures. This is not a supernova event; it is the prior phase that forms the white dwarf. The white dwarf core is usually composed primarily of carbon and oxygen. The collapse of the core is halted by electron degeneracy pressure. The electron degenerate matter, of which a white dwarf is composed, has its pressure determined by quantum rules that require that no two electrons can occupy the same state of position and momentum.

A Type 1a supernova is formed when a white dwarf of a particular mass undergoes a supernova explosion. It was shown in the 1930s by Chandrasekhar that the maximum mass supportable in the white dwarf state is 1.38 solar masses (1.38 times our Sun’s mass). In essence, at this limit, the electrons are pushed as close together as possible. If the white dwarf is near this limit and sufficient mass is added to the white dwarf, it will ignite thermonuclear burning of carbon and oxygen nuclei during a very rapid interval of a few seconds and explode as a supernova. The explosion is catastrophic, with all or nearly all of the star’s matter being thrown out into space. The supernova at maximum is very bright, for a while perhaps as bright as an entire galaxy. The additional mass that triggers the supernova is typically supplied by tidal accretion from a companion star found in a binary system with the white dwarf.

Because they all explode with the same mass, Type 1a supernovae all have more or less the same absolute brightness. This is key to their usefulness as standard candles.

In 1998 two teams of astronomers used these Type 1a supernovae to make the most significant observational discovery in cosmology since the detection of the cosmic microwave background radiation over 30 years earlier. They searched for these standard candle supernovae in very distant galaxies in order to measure the evolution and topology of the universe. Both teams determined the need for a non-zero cosmological constant, or dark energy term, in the equations of general relativity and with their initial results and others gathered later, its strength is seen to be nearly 3/4 of the total mass-energy density of the universe. These results have been confirmed by other techniques, including via detailed studies of the cosmic microwave background.

One needs two measurements for each galaxy to perform this test: a measurement of the redshift distance and a measure of the luminosity distance. The redshift distance is determined by the amount of shift toward the red portion of major identifier lines in the host galaxy’s spectrum, due to the expansion of the universe (the host galaxy is the galaxy in which a given supernova is contained.) The apparent brightness of the supernova relative to its absolute brightness provides the luminosity distance. Basically the two teams found that the distant galaxies were further away than expected, implying a greater rate of continuing expansion – indeed an acceleration – of the universe during the past several billion years compared to what would occur without dark energy.

What is exciting about the M101 supernova discovery last week is that it is so nearby, so easy to measure, and was caught very soon after the initial explosion. By studying how bright it is each day as the supernova explosion progresses, first brightening and then fading (this is known as the light curve), it can help us tie down more tightly the determination of the distance. This in turn helps to provide further precision and confidence around the measurement of the strength of dark energy.

References:

http://www.dailycal.org/2011/08/28/uc-berkeley-researchers-find-brightest-closest-supernova-in-years/

http://arxiv.org/abs/astro-ph/9812133  Perlmutter et al. 1999 “Measurements of Omega and Lambda from 42 High-Redshift Supernovae” Astrophys.J.517:565-586

 http://arxiv.org/abs/astro-ph/9805201  Reiss et al. 1998 “Observational Evidence from Supernovae for an Accelerating Universe and a Cosmological Constant” Astron.J.116:1009-1038

http://en.wikipedia.org/wiki/White_dwarf

http://en.wikipedia.org/wiki/Type_1a_supernova

http://en.wikipedia.org/wiki/Cosmic_distance_ladder


Dark Energy Drives Runaway Universe

Accelerating universe

Accelerating universe graphic. Credit: NASA/STSci/Ann Field

Dark energy was first introduced as a possibility as a result of the formulation of Einstein’s equations of general relativity. When he considered how the universe as a whole would behave under the general relativity description of gravity, he added a term to his equations, known as the cosmological constant. At the time the prevailing view was that the universe was static, and neither expanding nor contracting. The term was intended to balance the self-gravitational energy of the universe, and it thus acts as a repulsive force, rather than an attractive one. His basis for introduction of the cosmological constant was erroneous in two respects. The first problem is that the static solution was unstable, as if balanced on a knife edge. If you nudged it a little bit by increasing the matter density in some region slightly, that region would collapse, or if you lowered the density ever so slightly, that region would expand indefinitely. The second problem is that by 1929 Edwin Hubble had demonstrated the universe is actually expanding at a significant rate overall.

Subsequently, Einstein called the introduction of the cosmological constant his “greatest blunder”. After the realization that we live in an expanding universe, while the possibility of the cosmological constant having a non-zero value was sometimes entertained in cosmological theory, it was mostly ignored (set to zero). Over the next several decades, attention turned to better measuring the expansion rate of the universe and the inventory of matter, both ordinary matter and the dark matter, with the amount of the latter implied by long range gravitational effects seen both within galaxies and between galaxies. Was there enough matter of both types to halt the expansion? It seemed not, rather that there was only about 1/4 of the required density of matter, and that was mostly in the form of dark, not ordinary matter. Matter of either type would slow down the expansion of the universe due to its gravitational effects.

After 1980, the inflationary version of the Big Bang gained acceptance due to its ability to explain the flat topology of the universe and the homogeneity of the cosmic microwave background radiation, the relic light from the Big Bang itself. The inflationary model strongly indicated that the total energy density should be about 4 times greater than seen from the matter components alone. It is the total of energy and matter (the energy content of matter) which determines the universe’s fate, since E = mc^2.

In 1998 the astounding discovery was made that the universe’s expansion rate is accelerating! This was determined by two different teams, each of which were making measurements of distant supernovae (exploding stars). And it was confirmed by measurements of tiny fluctuations in the intensity of the microwave background radiation. The two techniques are consistent, and a third technique based on X-ray emission from clusters of galaxies, as well as a fourth technique based on very large scale measurements of relative galaxy positions, also give results consistent with the previous two techniques. The inflationary predictions are satisfied with dark energy presently three times more dominant than the rest mass energy equivalent from dark matter plus ordinary matter. Further measurements have refined our understanding of the relative strength of dark energy in comparison to dark matter and ordinary matter. The best estimates are that, today, dark energy is 74% of the universe’s total mass-energy balance.

In the cosmological constant formulation, dark energy is constant in time, while the matter density drops as the universe expands, in proportion to the cube of the scale factor. So if we consider the universe in its early days the energy contained in the dark matter would have dominated over dark energy, as the mass density would have been much greater than today. The crossover from matter dominated to dark energy dominated came after the universe was about 9 billion years old, or about 5 billion years ago. This emergence of dark energy as the dominant force, due to its nature as a repulsive property of “empty” space-time, results in an accelerating expansion of the universe, which has been called the “runaway universe”. Our universe is apparently slated to become hugely larger than its current enormous size.

Why is dark energy important then? Since five billion years ago, and on into the indefinite future, it has dominated the mass-energy content of the universe. It drives a re-acceleration of the universe. It inhibits the re-collapse (“Big Crunch”) of our entire universe or even substantial portions of the universe. Thus it naturally extends the life of the entire universe to trillions of years or much more – far beyond what would occur were the universe to be dominated by matter only and with density at the critical value or above. Dark energy thus works to maximize the available time and space for life to develop and to evolve on planets found throughout the universe.


Dark Energy Survey

DES logo

Dark Energy Survey logo

The Dark Energy Survey (DES) is a ground-based cosmology experiment led by astronomers from the US, Brazil and Europe. It has begun its trip to Chile where it is scheduled to begin observations in November, 2011 using the 4 meter Victor M. Blanco telescope in the Atacama desert. It uses a new highly sensitive camera design called DECam, with resolution totaling 570 Megapixels and employing very large pixels, and it emphasizes sensitivity in the red and infrared portions of the spectrum, in order to measure galaxies out to redshifts of 1 and beyond. Galaxies in the early universe are far away from us and have high redshift values. Light which they would have originally emitted in the blue or yellow portions of the optical spectrum has shifted toward the red or infrared, thus the emphasis on detection of infrared photons for this work.

The DES uses a 4-pronged attack to improve the measurement of the dark energy and other cosmological parameters. These 4 tests are:

  1. Supernovae – Type 1a supernovae are thought to occur when a white dwarf in a binary stellar system accretes mass from its companion. Once enough mass is accreted, the white dwarf is pushed over the Chandrasekhar limit of 1.4 solar masses, the gravity of the star’s mass overwhelms the pressure support from its ‘degenerate electron’ matter, and the white dwarf undergoes core collapse and becomes a supernova. It is temporarily as bright as an entire galaxy. Such a supernova can be detected at large distances (high redshifts) and very importantly, since the mass of the supernova is always the same, the absolute brightness of this type of supernova is essentially expected to be the same as well. This allows us to use them as standard candles for distance measurement and thus for cosmological tests.
  2. Baryon acoustic oscillations – This test looks at the statistics of galaxy separations at very large scales. In the early universe, sound waves were established in the hot dense plasma, reflecting pressure generated by the interaction of photons and ordinary matter. Dark matter does not participate except gravitationally. A “sound horizon” is expected with a present size of about 500 million light years, and this acts as a standard ruler as the universe expands. A bump in the correlation function, which measures the probability of one galaxy being near another, is expected at this characteristic distance.
  3. Galaxy cluster counts – This test of how many galaxy clusters are detectable versus redshift was apparently first proposed by myself in 1980, in the context of X-ray emission from the very hot diffuse gas found between galaxies in galaxy clusters. This approach offers certain advantages in comparison to simple galaxy counts versus redshift. In this case it will be performed in the infrared and red, observing the galaxies themselves. Galaxy clusters contain up to 1000 or more galaxies within a single cluster. The number of clusters that can be seen at a given redshift is dependent on the cosmological model and the mass of the cluster, since dark matter promotes cluster formation through gravitational attraction. Dark energy inhibits cluster formation, so this helps to measure the relative strength of dark energy at earlier times. The team expects to detect over 100,000 galaxy clusters, out to redshifts of 1.5.
  4. Weak lensing – This refers to gravitational lensing. This occurs when a source galaxy is behind an intervening galaxy cluster and the gravity of the cluster bends the light from the source galaxy in accordance with general relativity. By surveying a very large number of galaxies, a strong statistical measure of this bending, also known as cosmic shear, can be taken. The amount of shear will be measured as a function of redshift (distance). This shear is sensitive to both the shape of the universe and the way in which structure develops over time.

More info: http://www.quantumdiaries.org/2011/02/11/des-first-light-countdown-9-months-to-go-decam-on-telescope-simulator/

http://en.wikipedia.org/wiki/Dark_Energy_Survey

http://news.medill.northwestern.edu/chicago/news.aspx?id=182835


Foreword, by Rich Brueckner

“Through our eyes, the universe is perceiving itself. Through our ears, the universe is listening to its harmonies. We are the witnesses through which the universe becomes conscious of its glory, of its magnificence.”

— Alan Watts

We all know of the Big Bang, how our universe came to be in a massive explosion, seemingly starting from nothingness. And for those who study cosmology, further understanding requires us to define the dark energies that somehow endowed our world with order.

Now, we haven’t observed dark energy, dark matter, and the secrets of dark gravity directly, but we do see their effects. As we learn in this book, without them, the universe would not have formed in a way that could have spawned intelligent life.

As a writer, I am intrigued by these dark energies because they imply a backstory–phenomena that happened first that led to this outcome. So in this way, dark energies seem to me to be metaphors of science. Like the stories of Genesis and Adam and Eve, what they really represent is a deeper truth.

In this book, Dr. Perrenod does a wonderful job of explaining the origins of the universe in way that is accessible to the layman. When you want to understand how the universe came to be, you ask an astrophysicist. But when you really want to know why, I think you have to start by asking yourself some questions. Try a thought experiment.

Put yourself in the place of a Universal Mind before the Big Bang. If you really wanted to understand yourself, you would need to have something intelligent outside of yourself that could experience that which is you. Not to get metaphysical here, but if we were at the scene of a crime, what I’d be suggesting here is motive.

Thanks to modern physics and cosmology, we no longer live in a universe where dark forces lurk far beyond our capacity for comprehension. I believe that, through the works of Stephen Perrenod and others, we will come to that knowing. But even as we look out to the stars, I think it begins with understanding that not only are we within the universe, but the universe is within us.

Rich Brueckner is President of insideHPC.com