Thursday, October 27, 2011

Dark Matter, Dark Energy and the Runaway Universe

In a spiral galaxy, the ratio of dark-to-light matter is about a factor of ten. That's probably a good number for the ratio of our ignorance-to-knowledge.  We're out of kindergarten, but only in about third grade.

Vera Rubin (1975)


When the Nobel Prizes in Physics for 2011 were announced earlier this month, I just happened to be reading a book titled "The 4% Universe – Dark Matter, Dark Energy and The Race to Discover the Rest of Reality" written by Richard Panek. Also by a strange coincidence, I happened to be reading the particular section of the book describing the works of the three cosmologists, Saul Perlmutter, Brian Schmidt and Adam Riess, who ended up sharing the award for their mind boggling and remarkably unexpected discovery concerning the ultimate fate of the Universe, made at the turn of the last millennium.  I wonder if I was subconsciously anticipating the award.

I devote this post to a description, as far as possible in nontechnical language, of the key astrophysical and cosmological concepts leading up to this discovery, some of the more important related discoveries leading up to it and their implications to an understanding of the nature of the Universe.  In doing so I will naturally be indulging in oversimplification, sacrificing much of the rigor associated with such advanced concepts in a frontier discipline like Cosmology.  Interestingly, the whole story is less than a century old.

Unlikely as it may sound, some of the basic concepts of contemporary physics and astrophysics central to this story have been touched upon in my two previous blog posts titled "Bose and Einstein – A Historic Collaboration (Jul 11)" and "Chandrasekhar – Fermi-Dirac and White Dwarfs (Aug 11)", respectively.  Readers without a background of collegiate physics may find it helpful to read them first before continuing with the present one.  Even if they don't understand all of what I am trying to convey, my purpose will be served if I am successful in generating a little bit of the excitement that is sweeping the scientific community today. Incidentally, the whole story is intricately woven around the edifice of Einstein's General Theory of Relativity.

Beyond the Milky Way

Less than a century ago, our view of the Universe did not stretch beyond the familiar Milky Way and numerous objects presenting a fuzzy appearance were classified as nebulae and believed to be part of the Milky Way.  However, bigger and better instruments like the 100" Mount Wilson telescope in California and vast refinements in techniques of spectroscopic and photometric analysis, showed that most of these nebulae were actually far away galaxies much like our own Milky Way, a discovery we owe principally to the great Edwin Hubble.  A typical galaxy has about as many stars in it as there are galaxies in the Universe (this number is about a hundred billion).  For example, the naked-eye fuzzy object in Andromeda constellation, formerly thought of as a nebula, was shown to be a galaxy much like our own Milky Way and situated about 2.2 million light years away.  A light year is the distance travelled by light through free space in one year, travelling at the constant speed of 300 000 km/sec, and is equal to 9.46 trillion kilometers.  This may be an unimaginably large distance by earthly standards, but quite puny on the cosmic scale.  The Andromeda galaxy is indeed our nearest galactic neighbor among a galactic population of hundreds of billions (I am ignoring two satellite galaxies called the Large and Small Magellanic Clouds, which are much closer and can be seen in the southern skies).  Our Sun is an insignificant star in an equally insignificant part of the Milky Way; and the Earth is, but for the intelligent life it harbors, a rather insignificant part of the solar family.

Measuring Galactic Distances

The world of astronomy and astrophysics boasts of some great female personalities, one of them being Henrietta Swan Leavitt of the USA, whose painstaking and pioneering work in 1908 led to the development of what is called the 'standard candle' for the measurement of distances on the galactic scale.  She discovered a remarkable class of stars of periodically varying brightness, known as cepheid variables, which exhibit a precisely predictable relationship between their intrinsic brightness and periodicity.   This means, if we measure its variation in brightness from one peak value to the next (this is easy to do with a photometer), we can work out how bright it actually is.  If the light from it is bright enough to measure (as can often be achieved through powerful telescopes), we get its apparent brightness. A measurement of its periodicity yields its actual brightness. Using the inverse square law of variation of brightness with distance, it is then possible to work out its actual distance.

If we want to know how far away a particular galaxy is, we need to discover at least one cepheid variable star within it and apply this technique. This has been done successfully with almost all nearby galaxies.  However, if a galaxy is too far away (typically billions of light years away) the technique won't work because the individual stars cannot be resolved sufficiently to look for variable ones.  As we shall see later, the three cosmologists winning this year's Nobel Prize for Physics solved this naughty problem by studying another class of stellar objects called supernovae, which can be discovered even within such far off galaxies, and developed another 'standard candle' for the measurement of very large inter galactic distances.
Measuring Galactic Speeds

All objects in the Universe are in constant motion, with different speeds and in different directions with respect to each other at any instant of time.  These parameters associated with any stellar object are as important as its distance from any observer.  A simple and well known principle of physics, called the Doppler Effect, enables us to determine them through some straightforward observations.

To understand the Doppler Effect, let us recall a common experience most of us would have had.  This is the distinct difference in the pitch (frequency) of the sound emanating from a railway engine hooting as it is passing by us at a fair speed near a railway track.  As it is approaching us the pitch sounds distinctly higher than as it recedes from us.   When compared to the pitch from a stationary engine, the pitch from the receding engine is reduced and the one from the approaching engine increased.  In other words, the wavelength (which is the inverse of the frequency or pitch) of the sound note from the object moving away from us increases and that from any object approaching us decreases.  The magnitude of the change is related to the speed of the object; by measuring it we can also measure the speed of the object.
The Doppler Effect is also applicable to the light (or any electromagnetic radiation) emitted by any stellar object.  By using the Doppler formula one can determine the speed of the object by measuring the change in wavelength of a suitable spectral line in the radiation emitted by the object using a spectroscope attached to the viewing end of a telescope.   The change is of course with reference to the same spectral line from a suitable terrestrial source which is at rest with reference to the measuring equipment.

Most Doppler measurements of the speeds of stars and even nearby galaxies in the early phase of such experiments were consistent with the expectation of a random distribution of such speeds.  However, as bigger and better telescopes came into use and measurements could be attempted on the more distant galaxies, a discernible pattern started emerging, mainly through the efforts of Edwin Hubble who did his measurements with what was at that time the world's largest telescope, the 100" Hooker Telescope on Mount Wilson in California.   He observed that most of these distant galaxies showed a 'red shift', meaning that their spectral lines systematically showed an increase in their wavelengths.  Clearly, they were moving away from us at speeds that could be determined from the measured shifts.  This finding was highly unexpected and sensational by itself.  However, an even greater sensation was to follow when it was noticed that the farther away a galaxy was the greater its red shift, i.e. its speed away from us.  The effect was readily quantifiable.  The speed was directly proportional to the distance!  For some reason, the whole Universe appeared to be expanding in a rather simple way, at least when looked at on a sufficiently large scale.

The Big Bang Universe

Hubble's discovery of distant galaxies speeding away from us, and from each other, at ever increasing speeds, could be interpreted only one away – that they must have started off on their journeys close together once upon a time in the distant past.  Arguing on this line, the Belgian astronomer-cum-catholic priest Georges Lemaitre, who had worked with Arthur Eddington in Cambridge, suggested that all the galaxies and by implication all the matter in the Universe must have originated from a 'primordial atom', marking the very birth of the Universe. Significantly, he was not envisaging the 'creation' of the Universe in the biblical sense, but purely as a theoretical construct to fit the observations.  As the idea developed further, principally by Alexander Friedmann, Ralph Alpher and the charismatic George Gamow, the entire Universe was visualized as an incredibly dense, super hot and compact primordial entity which, for some as yet unclear reason, blew up in a gargantuan explosion, started expanding very rapidly and in due course ended up in the form that we find today. On the basis of the measured speeds of the receding galaxies the process was estimated to have started about twenty billion years ago. Details of all that happened from time zero till to date needed to be worked out, but the basic idea sank in and attracted attention as an alternative to the prevailing 'steady state' theory which visualized the Universe as without a beginning or an end and without any mad rush among its inhabitants to run away from each other.  Fred Hoyle, who had championed the steady state theory and was unimpressed by the new idea of an expanding Universe with a definite beginning in time, called the latter disdainfully as the Big Bang and the name has stuck ever since.
Einstein's Blunder

The concept of an expanding Universe with a definite beginning had been an unwelcome consequence of the General Theory of Relativity. Einstein had derived an equation for the behavior of the four dimensional space-time continuum in which gravitation was interpreted as a geometrical property of the continuum, with matter producing a distortion or curvature of it to enable objects to move in it the way they are observed to move.  An expanding Universe was a logical consequence of such an exercise. However, Einstein's belief in a static, steady state Universe was so strong that he tweaked his equation to conform to the concept by arbitrarily introducing an extra term called the cosmological constant into his result. Later, when Einstein realized that the theoretical foundation for the expanding Universe had been staring at his face all the time and that he had lost out on what was perhaps the most important discovery in cosmology up to that time, he termed the introduction of the cosmological constant as the biggest blunder of his life.

Cosmic Microwave Background (CMB)

The observed accelerations of far away galaxies did provide a very sound argument for the Big Bang theory but more was needed before it could supplant the steady state theory and take root as the definitive theory of formation and evolution of the Universe.  Such a 'smoking gun' was provided serendipitously in 1965 by two Bell Telephone Labs researchers in the USA who were using a horn antenna for detection of annoying radio interferences from nearby sources.  A systematic observation led the two, Arno Penzias and Robert Wilson, to the totally unexpected and puzzling discovery of a uniform but faint microwave background radiation coming from everywhere in space.  The radiation was interpreted by Robert Dicke, David Wilkinson and others at the nearby Princeton University as left over from an early stage in the evolution of the Universe, and its discovery was considered a crucial test of the Big Bang theory of the Universe. From the measured wavelength and energy distribution of this cosmic microwave background (CMB) radiation and using Planck's radiation formula which links them to temperature, they could determine the temperature of the material emitting the radiation to be around 3K (three degrees on the Kelvin scale, which corresponds to about minus 270 degrees on the familiar Celsius scale).  This could be interpreted as the mean temperature to which the Universe has cooled today starting from an extremely hot and dense phase around 20 billion years ago.  With some assumptions it was possible to estimate the temperature at which the whole process should have started and the rate at which the cooling has proceeded since then.  The starting temperature should have been so enormously high that the 'primeval atom' should have consisted only of pure energy, with particles materializing from the energy at a slightly later, cooler epoch.  From then on the formation of nuclear matter, synthesis of higher elements, atoms, molecules, etc., should have all followed according to the laws of physics which are now well understood.

The highly uniform temperature of the CMB radiation over the entire sky strongly supports the Cosmological Principle according to which the Universe is homogeneous and isotropic when averaged over very large scales.

The discovery of the cosmic microwave background radiation proved such an important cog in the wheel of the Big Bang and the subsequent expansion of the Universe that it won a Nobel Prize for Physics in 1978 for Penzias and Wilson.  The Big Bang theory found its footing and the intellectually satisfying steady state theory began to fade away into oblivion, unable to withstand the assault from observational evidence, much to the annoyance of people like the flamboyant Fred Hoyle and his Indian associate Jayant Narlikar.

The Evolving Universe

Our present-day understanding of the evolving Universe based on the Big Bang model rests on a rock solid foundation of observational evidence provided by telescopes and accessories of ever increasing sophistication, ranging from the now defunct Hooker telescope used by Hubble to NASA's immensely useful space telescope appropriately named after Hubble himself, still very much in use.  In between, we have had a number of progressively larger and more advanced ground based telescopes in different parts of the world. Tools and techniques of Information and Communication Technology have played a crucial role in the process.  For example, the CCD camera which is so ubiquitous in hand held electronic devices today has revolutionized astronomical imaging techniques beyond belief.
Without pausing to address the crucial question of how, let me summarize what we know about our Universe today.  However bizarre the ideas may appear, they are all backed by strong observational evidence.  Incidentally, when we talk of an expanding Universe we mean that space itself is expanding, carrying with it the galaxies like spots on the surface of an inflating balloon, in a manner consistent with Einstein's General Relativity.  Also, a question like what is it expanding into is meaningless since by definition there can be nothing outside the Universe.

The Big Bang event is now reliably determined to be 13.7 billion years old, not 20 billion as initial estimates indicated.  It all began in a blinding flash lasting the tiniest fraction of a second, smaller than any time duration encountered in any physical process, during which the primordial atom underwent an incredibly large and exponential expansion to set the whole evolutionary process in motion.  This has come to be called the inflationary phase of the Universe and the concept was propounded by Alan Guth of Stanford University to account for the extremely uniform distribution of the CMB radiation and several other puzzling observations.

When inflation stopped, the Universe consisted of what is called a quark-gluon plasma from which elementary particles were formed. At the prevailing extremely high temperatures, and the predominance of radiation over matter, particle-antiparticle pairs of all kinds were being continuously created and destroyed in collisions. At some point in time a certain imbalance (the precise nature and reason for which is yet to be understood) set in, giving rise to a very small excess of matter particles over antimatter particles.  This resulted in the predominance of matter over antimatter in the present Universe. As the Universe continued to grow in size and drop in temperature, the particle energies decreased to a level at which protons and neutrons could be synthesized.  At this time the energy density of the Universe was dominated by photons.

A few minutes into the expansion, when the temperature was about a billion degrees Kelvin, neutrons combined with protons to form deuterium and helium nuclei in a process called Big Bang Nucleosynthesis.  However, about 75% of protons remained as hydrogen nuclei. As the Universe cooled further, the effects of matter began to dominate those of radiation and gravitation came into the picture very prominently.  After about 380,000 years, the electrons and nuclei combined into hydrogen atoms, which became the dominant form of matter at that time.  Radiation got decoupled from matter and spread out through space uniformly, largely unimpeded. This left-over radiation is what constitutes the CMB today.

Over a long period of time, the slightly denser regions of the nearly uniformly distributed matter attracted other nearby matter gravitationally and thus grew even denser.  This clumping of matter should have produced small irregularities in the otherwise uniform distribution of matter on a cosmic scale. They have in fact been discovered through correspondingly minute, local anisotropies (typically about one part in a hundred thousand) seen in the intensity of the CMB radiation, most prominently and conclusively by the path breaking WMAP (Wilkinson Microwave Anisotropy Probe) satellite.  The process resulted in the eventual formation of gas clouds, stars, galaxies, and the other stellar objects, including clusters and super-clusters of galaxies, populating the Universe today. The details of this process depend on the amount and type of matter in the Universe and are only now beginning to be understood. However, the type of matter with which we are familiar at both the atomic and cosmic scales is grossly, in fact hopelessly, inadequate for the purpose.  Observational evidence, including the one provided by WMAP, supports only a little over 4% of the Universe as consisting of this 'normal' matter; we are literally in the dark as regards the rest of it.

Dark Matter

Almost everything we understand about normal matter is through an analysis of the electromagnetic radiation we receive from such matter.  For matter on a cosmic scale, this is strongly complimented by a study of the gravitational forces among its constituents. This is how we know the behavior and physical properties of stars, stellar systems, interstellar matter, galaxies, inter galactic matter, galactic clusters and super clusters, etc.  As we shall now see, a large part of the matter on the galactic and super galactic scale behaves in a manner consistent fully with the existence of gravitational forces among its constituents, but without any electromagnetic interaction and therefore invisible in the normal sense, giving rise to the concept of dark matter.

It is time to bring in another great female astronomer, Vera Rubin, to whom we owe the earliest recognition of dark matter. Rubin and her colleague Kent Ford analyzed a large number of galaxies to study the radial distribution of matter in them using the Doppler principle outlined earlier. The stars in the disk of a galaxy move in roughly circular orbits around the center. If the disk is inclined to our line of sight, the stars on one side are approaching us while those on the other side are moving away. The wavelength shift is proportional to the speed of the light source relative to us.  Rubin and Ford made careful measurements of Doppler shifts and then calculated the orbital speeds of the stars in different parts of those galaxies.

Because the core region of a spiral galaxy has the highest concentration of visible stars, astronomers assumed that most of the mass and hence gravity of a galaxy would also be concentrated towards its center. In that case, the farther a star is from the center, the slower its expected orbital speed. Similarly, in our solar system, the outer planets move more slowly around the Sun than the inner ones. By observing how the orbital speed of stars depended on their distance from the center of a galaxy, one could calculate how the mass is distributed throughout the galaxy.  To their great surprise, Rubin and Ford discovered that the stars from the sparsely populated outer parts of the galaxy were moving just as fast as those in the interior.  This was very odd, because the visible mass of a galaxy does not have enough gravity to hold such rapidly moving stars in orbit. It followed that there had to be a tremendous amount of unseen matter in the outer regions of galaxies where the visible stars are relatively few.

Independent evidence for the existence of such dark matter on the galactic scale has also been discovered through a technique called gravitational lensing. A gravitational lens is formed when the light from a very distant, bright source, such as a quasar, is 'bent' around a massive object (such as a cluster of galaxies) between the light source and the observer.  The bending attributable to virtually invisible objects in the foreground indicates the presence of dark matter on a vast scale.

While there is ample evidence for dark matter on the cosmic scale, there is as yet no evidence for it at the local level.  We have no idea about what dark matter is made up of, in sharp contrast to our understanding of ordinary matter through a wide range of inter particle interactions and the interactions between material particles and electromagnetic radiation.  Whatever entities dark matter may be made up of, they don't interact with ordinary matter, at least to a directly observable degree.  They appear to be as elusive as the neutrinos, which have been incidentally ruled out as possible dark matter candidates since they are virtually massless.  Dark matter particles need to be quite massive even if very illusive.  All efforts to detect such particles in laboratory conditions all over the world have so far proved futile; but they are continuing, actually on an expanded scale.  In anticipation of its eventual discovery the name WIMP (Weakly Interacting Massive Particle) is already in use for the hypothetical dark matter particle.  The objects identified as consisting of dark matter in galaxies and galactic clusters are being described as MACHOS (Massive Astrophysical Compact Halo Objects).

Structure and Properties of the Universe

As the observational evidence piled up in favor of the Big Bang and the expanding Universe, the obvious question in everyone's mind was what would happen to it in a sufficiently distant future.  Would the expansion continue indefinitely into the future, with the Universe ending up in a big whimper or would it slow down, come to a halt and eventually fall back on itself under the influence of gravity, leading to a big crunch?  The answer depends on the dynamic properties of space-time itself, determined by the interplay between matter and gravitation. It also depends on the topological (geometrical) properties of space itself as could be determined by general relativity, supported by observations.

The outward expansion of the Universe is countered by an inward pull of gravity on the expanding matter and this in turn depends on the density and pressure of the matter in the Universe.  If the density of the Universe exceeds a certain critical value, then the geometry of space is closed and positively curved like the surface of a sphere.  If the density is less than the critical value, then the geometry of space is open (infinite) and negatively curved like the surface of the saddle placed on a horseback.  If the density of the Universe is nearly equal to the critical density, then the geometry of the Universe is flat, like a sheet of paper.  Based on WMAP measurements, we now know that the Universe is flat with only a 0.5% margin of error.   This is a very significant finding, with a bearing on the eventual fate of the Universe.

Do we have a good estimate of the critical density of the Universe?  Yes, we do and the value is the equivalent of there being just about six protons (hydrogen nuclei) in one cubic meter of space!  Remember, only a little over 4% of this is our familiar normal matter! This is so incredibly small that it is impossible to produce a vacuum of this magnitude on Earth no matter how we try.
Evolution Scenarios

The diagram below depicts different scenarios for the eventual fate of the Universe in the tug-of-war between the outward momentum of expansion and the inward gravitational pull.

If the density of the Universe is greater than the critical density, then gravity will eventually win and the Universe will fall back on itself, giving rise to the hypothetical 'Big Crunch' as indicated by curve A.  If the density of the Universe is less than the critical density, then the Universe will expand for ever as indicated by curves B and C.  Gravity might slow the expansion rate, but there isn't enough gravitational pull from the material in the Universe to stop or reverse the outward expansion.  This is described as the 'Big Chill' or the 'Big Freeze' because the temperature continually decreases with the passage of time.  However, available evidence actually supports an accelerated expansion scenario as depicted in curve D for reasons soon to be discussed.

Supernovae as Standard Candles

As they reach the end of their life cycles, super massive stars may use up their nuclear fuel and collapse under their own weight.  The collapse leads to an incredibly gigantic explosion, sends out a shock wave through space followed by a shell of material ejected at great speeds from the star's atmosphere.  This is accompanied by the release of enormous amounts of radiation in a blinding flash that lasts a very short period, followed by rapidly decreasing brightness over a period of just a few weeks.  Called a supernova, this is a relatively rare event within any galaxy, on the average only one in a hundred years or more.  At its peak, a supernova can outshine the entire galaxy within which it occurs.  For this reason it is relatively easy to detect and study a supernova even within the most distant galaxies.
Depending on the details of the mechanism of energy production and release, supernovae come in two categories – Type I and Type II.  Within each type there are subdivisions. Our story is centered round Type Ia supernovae whose brightness variations show a regularity that makes them uniquely suitable as yardsticks for measurement of distances to the farthest galaxies known.  They can be used as 'standard candles' much like the cepheid variable stars for much shorter distances.  This is the technique largely pioneered, perfected and employed extensively with startling consequences by Perlmutter, Schmidt and Riess who have been awarded this year's Nobel Prize in Physics. Besides using a number of some of the largest earth based telescopes, they also made substantial use of the Hubble Space Telescope which provided some major advantages for the purpose.  Indeed, their discovery is recognized as one of the many outstanding achievements of the Hubble telescope itself.

A Type Ia supernova results from having a white dwarf star in a binary system.  Because of the huge gravitational force of the dwarf star, matter is sucked in continuously from the normal star to the white dwarf until the latter attains a critical mass (the Chandrasekhar limit) and undergoes a thermonuclear explosion.  Because all white dwarfs achieve the same mass before exploding, they all achieve the same luminosity and can be used as standard candles.  By observing their apparent brightness, the actual distance can be determined by using the inverse square law for brightness variation with distance. By knowing the distance to the supernova, we know how long ago it occurred. By measuring the red shift from the spectrum of the supernova, we can determine its speed of recession and hence how much the Universe has expanded since the explosion using Hubble's law.
Despite the rarity of the supernova event in any galaxy, potentially a good number of supernovae can be studied at any one time because of the enormous number of galaxies existing in the Universe.  By doing so, we can piece together an authentic history of the expanding Universe extending to the very edge of space.

Dark Energy

In the 1990's two teams of astronomers, working independently but in frequent touch with each other, the Supernova Cosmology Project (CSP) based at the Lawrence Berkeley National Laboratory led by Perlmutter and the (international) High-Z Supernova Search Team led by Schmidt in Australia, investigated a large number of distant Type Ia supernovae in order to measure the expansion rate of the Universe. Consistent with wide-spread belief, they expected that the expansion would be slowing down (according to one of the scenarios A, B or C indicated in the previous diagram).   They expected the supernovae to be actually brighter than indicated by their measured reds shifts.  Instead, to their utter surprise, bordering on consternation, they found the supernovae to be actually fainter than expected. The brightness of the supernovae decreased with distance significantly more rapidly than anticipated.  There could only be one interpretation of this. The expansion of the Universe was accelerating, as in scenario D of the diagram!

As outlined earlier, measurements of the CMB radiation indicate that the Universe has a flat geometry on a large scale. The amount of matter in the Universe, either normal or dark or taken together, is very much less than what is required to produce this flatness. The difference must therefore be attributed to some hitherto unknown cause, which came to be called dark energy for want of a better description. It is this dark energy which is seen to be causing the observed accelerated expansion of the Universe by countering the effects of gravity; in other words acting like anti-gravity.

Evidence for the existence of dark matter is now clear-cut though its nature is still very elusive.  By contrast, dark energy remains a complete mystery. The name dark energy implies that some kind of 'stuff' must fill the vast reaches of mostly empty space in the Universe in order to produce an accelerated 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 and very well understood. But this analogy breaks down completely when it comes to dark energy.

Some astronomers now tend to identify dark energy with Einstein's discarded cosmological constant which he had originally introduced when he saw that his theory was predicting, embarrassingly at that time, an expanding Universe.  If successful, this idea may provide a backdoor vindication of Einstein's action, but for entirely unforeseen reasons.

While bright ideas to account for dark energy and its effects are not wanting, for the present at least its nature is a mystery waiting to be unraveled as in any good detective story.

The 4% Universe

Just half a century ago we were blissfully ignorant of the fact that the Universe we were then familiar with was only about four percent of the 'real' one that we have just begun to recognize.  The rest of it was, and still is, hidden from our direct experience. Vera Rubin's observations about our level of ignorance quoted at the beginning are perhaps as valid today as they were when she made them.

It is now reliably estimated that the Universe consists of about 23% dark matter besides the 4% normal matter.  This leaves out a huge 73% of it to be accounted for by dark energy which is as elusive today as when first recognized.  Unraveling the nature of dark matter and dark energy, which are so overwhelmingly pervasive in the Universe, is easily the greatest challenge facing astrophysics and cosmology today.

The Runaway Universe

It is now evident that we are part of a Universe that is behaving like a wildly galloping horse, running away from everything aimlessly and without any tangible purpose, though governed by all known laws of physics.
The following illustration from a NASA website depicts in a nutshell the evolution of the Universe from the Big Bang to the present.  The accelerated expansion started about 7.5 billion years ago; until then the expansion was nearly in conformity with our earlier expectation.  This is because gravity was still a dominant force up to that epoch.

The Ultimate Fate

The Earth is an insignificant part of the solar family which occupies an equally insignificant place in the Milky Way galaxy.  Less than a century ago we discovered that this is itself an utterly insignificant part of the Universe as we came to know.  Now we know that the Universe we have been able to understand so far is only an insignificant part of the one that we are yet to understand.  This sums up not only the level of our ignorance but also the degree of our insignificance. The vast Universe is still waiting to be understood.  But we do seem to have at this time a plausible answer to one of the ultimate questions bothering humankind for centuries – what is the ultimate fate of the Universe?  Cosmology is no longer an infant discipline depending on philosophy and metaphysics to provide the answers. It is now a full blown science that can face this question head on.

All available observational evidence today points to a runaway Universe in a state of wildly accelerating expansion, with its hidden dark energy tearing it apart at the seams. If the process continues, the galaxies should move so far away as to eventually become invisible to us by any means and we might at best be able to see our nearest neighbors.  Worse still, the galaxies should be torn into shreds like confetti, even the matter inside them being ripped apart into its constituent particles, perhaps into a quark-gluon plasma of the primordial Universe, but spread over incredibly vast distances.  We can however take solace in the comforting thought that this is all far off in the future, hundreds of billions of years away!