6 February 2013
The Age of The Universe
Professor Carolin Crawford
The idea that the Universe might have an age is a relatively new concept, one that became recognised only during the past century. Even as it became understood that individual objects, such as stars, have finite lives surrounded by a birth and an end, the encompassing cosmos was always regarded as a static and eternal framework. The change in our thinking has underpinned cosmology, the science concerned with the structure and the evolution of the Universe as a whole. Before we turn to the whole cosmos then, let us start our story nearer to home, with the difficulty of solving what might appear a simpler problem, determining the age of the Earth.
Age of the Earth
The fact that our planet has evolved at all arose predominantly from the work of 19th century geologists, and in particular, the understanding of how sedimentary rocks had been set down as an accumulation of layers over extraordinarily long periods of time. The remains of creatures trapped in these layers as fossils clearly did not resemble any currently living, but there was disagreement about how long a time had passed since they had died.
The cooling earth
The first attempt to age the Earth based on physics rather than geology came from Lord Kelvin at the end of the 19th Century. Assuming that the whole planet would have started from a completely molten state, he then calculated how long it would take for the surface layers of Earth to cool to their present temperature. Unfortunately his estimation that the cooling process would last less than 400 million years strongly conflicted with the timescale of billions of years that was by then deemed necessaryfor change through natural selection to have taken place, in Darwin’s new idea of evolutionary biology.
We now understand that Kelvin’s estimate was too low because the Earth does more than just cool: he was unaware of geological processes that continually warm planetary surface, such as convection within the Earth that can transport heat away from the interior to the outer layers, and the energy released in the process of radioactive decay.
Radioactivity and Radiometric Dating
Radioactivity was discovered shortly after, at the turn of the 20th century. Not only does radioactivity release energy and slow the rate at which we can expect the Earth to have cooled, but it also supplied a new, and much more accurate method for dating the rocks through the process of radiometric dating. Isotopes of elements are chemically identical to the ‘normal’ version of the atom, but they contain a different number of neutrons within their nucleus. Usually an atom of an isotope which is unstable will spit out particles (protons, neutrons, alpha particles…) until it reaches a more stable configuration of the same element, or it breaks down into another, lighter and more stable element. This is the process of radioactive decay, liberating material and energy at each step. The rate at which a radioactive element undergoes such a transformation can be measured in the laboratory and it is characterised by the ‘half-life’; this is defined as how much time is required for half a mass of that substance to change. Radioactive decay occurs at a constant rate for any particular element, and is not affected by any external environmental factors such as pressure or heat. Half-lives range from fractions of seconds to millennia, and it’s elements with the latter - such as isotopes of uranium and thorium - which are of particular interest for dating the Earth. With half-lives of around 4.5 billion years, they persist in the Earth’s crust to the current day. It is thus possible to measure the age of Earth by determining the relative proportions of radioactive materials in geological samples. For example, uranium (U-238) decays into lead (Pb-206), and so over time, a sample of rock will contain less and less of the uranium and more of the lead. By measuring the concentration of the stable end product of the decay in a sample, and knowing the half-lives of the unstable elements, one can thus calculate the age of the rock (albeit with the judicious estimate of the initial makeup of the sample). Of course, the process of radiometric dating is further complicated by the way that radioactive elements don’t usually decay directly into stable elements, but into other radioactive elements with their own half-lives, and have to pass through many such stages en route to a nonradioactive end… Nonetheless, a lower limit to the age of the Earth can be assumed to be that of the oldest terrestrial rock.
Radiometric dating was not very successful at first, mainly because all the various stages in the chain of radioactive decay were still not well known or established, there was difficulty in measuring very long half-lives in the laboratory, and experimental uncertainty about the amount of lead contained in rock samples. It is also hard to find pristine samples of rock that are unaffected by geological processes such as plate tectonics, convection and volcanoes which can mix up rock between the crust, mantle and core. The British geologist Arthur Holmes persevered despite such difficulties, and using rock samples collected from all over the world, by the early 1920’s his radiometric dating results had established that the Earth was at least a few billion years old.
Age of the Solar System
Nowadays radiometric dating is a precise technique, and can also be applied to samples of material that have not undergone the geological change, or weathering by water and atmosphere that we have experienced on Earth. Much more pristine geological samples are available.
Of the Moon…
Specimens of moonrock returned by the Apollo missions from the most heavily cratered (and thus expected to be the oldest) regions of the Moon yield an age of 4.5 billion years for the lunar surface.
Even more primitive samples than from the Moon are available. Meteorites are examples of space rock handily delivered directly to Earth’s surface by gravity, and represent samples of the Solar System which have not experienced any geological processing at all. Radiometric dating of meteorites was first carried out in the 1950’s by the American geochemist Clair Patterson. Modern dating of elements in meteorites indicates a range of ages between 4.53 to 4.58 billion years (with an average 4.55 billion). Some of these meteorites incorporate very pristine parts of the early solar nebula, sampling material from a phase before the gas and dust had condensed into protoplanets.
And other objects in the universe?
Radiometric dating can’t, however, be easily applied to objects outside of the Solar System. Very high quality data are required, and so far the abundance of Uranium-238 has only been measured in a handful of individual stars at the outskirts of the Milky Way (yielding ages of 12.5±3 billion years). In larger, brighter systems such as a galaxy, all the elements are mixed up between gas and dust, and new isotopes of elements are being continually released to the mix by supernovae explosions at the end of massive stars lives.
The Age of the Sun
We can safely infer the Sun to predate the meteorites, but even establishing the lifetime of the Sun was not originally straightforward, as it was intricately linked to the problem of accounting for the source of the Sun’s energy. The fact that fossils had been found from hundreds of millions of years ago implied to Victorian scientists that the Sun had to be at least this old, as it could be assumed that past lifeforms also relied on sunlight as much as today’s. However, it was unclear what source of energy could sustain and produce sunshine for so long.
Lord Kelvin again came up with one of the early physical ideas, in collaboration with the German scientist Hermann von Helmholtz in the mid-19th Century. They proposed that the Sun shines because it shrinks – specifically, that the enormous weight of the outer layers of the Sun would squeeze the interior gases, and the gravitational compression would cause an increase in temperature to the point where the gas in the Sun would be hot enough to radiate. But there is insufficient mass in the Sun for this process to provide energy more than a few tens of millions of years, even with the later incorporation of the warming energy released by radioactive decay. The Sun requires a completely different source of energy.
Ordinary chemical burning of fuel is nowhere near efficient enough, as it would consume all the mass of the Sun within around 10,000 years. A proper understanding of the Sun’s energy source came only after Einstein’s 1905 special theory of relativity suggested a similar, but more efficient process. The famous equation of mass-energy equivalence, E=mc 2, shows that a tiny amount of matter can be converted into a tremendous amount of energy (because the speed of light, rendered as c in this equation, is such a large quantity). In the 1920’s the English astronomer Sir Arthur Eddington realised that the temperatures and pressures at the heart of the Sun are so high that the conditions are suitably dense and hot enough for nuclear reactions to take place. When hydrogen nuclei fuse together to form helium, a tiny amount of mass is liberated, which is then transformed into an enormous amount of energy. It took many more decades of both theoretical and observational work to fully refine our understanding of the fusion processes within stars. But it is straightforward to compare the current energy output of the Sun with how much material it has left available for nuclear fusion, leading to estimates for its lifetime that slightly predate the age of the Solar System – 5 billion years. Even if only 10% of all the Sun’s mass is hot enough to undergo nuclear fusion, it can continue at its current luminosity for at least 10 billion years.
So far in our story the longest timescale we have reached is the 5 billion years that the Sun has existed. This gives a clear minimum age the wider galaxy and cosmos. But how do we determine the age of the whole Universe? This is where we turn to cosmology.
The Eternal Universe
By the beginning of the 20th Century scientists were seeking to establish the finite age of both the Earth and the Sun. But there was no conception that the wider Universe was anything other than unchanging, infinite and eternal. This viewpoint had persisted for four centuries as an obvious consequence of Sir Isaac Newton’s laws of gravity. The Universe that we observe spread out before our eyes could only be interpreted as an eternal and infinite expanse of stars, as this was the only way to balance out the gravitational forces in every direction and hence prevent everything coalescing together.
Einstein’s Cosmological Constant
Even once Einstein’s theories had revolutionised our view of gravity as distortions created in space and time caused by the presence of a mass, the fact that it acted as an attractive force still seemed to contradict the observation of a spatially dispersed Universe. Indeed, Einstein struggled to come up with a static solution to his equations, resorting in the end to the addition of a ‘fudge factor’ known as the cosmological constant to his equations of general relativity. This introduces a kind of ‘anti-gravity’, or outwards pressure, that is able to balance gravitational attraction on very large scales, and keep the Universe static and uncollapsed.
Other theoretical astronomers – such as the Russian meteorologist Alexander Friedmann and the Belgian priest Georges LeMaitre were less concerned than Einstein about conforming to the scientific prejudice of the time. Independently each experimented with altering Einstein’s equations by varying the value of the cosmological constant to produce alternative versions of the Universe that could change with time from a young Universe to that of the present day. Not only could such a Universe then presumably also evolve further into the future, but LeMaitre in particular developed the idea that some solutions implied an apparent starting point to the cosmos, the ‘primeval atom’. Both scientists had their ideas and models firmly rejected by the scientific establishment of the time, including by Einstein. Full acceptance of their radical theories came not from theoretical work, but from the observations made by astronomers with access to a new generation of large telescopes.
The Expanding Universe
Edwin Hubble’s observations.
Edwin Hubble had already distinguished himself by establishing the distances to ‘spiral nebulae’ in the sky, showing beyond a doubt that they lay outside of the confines of the Milky Way, and thus that our Galaxy was just one of many ‘island Universes’ in existence. The determination of their distances was far from trivial, and itself relied on Henrietta Leavitt’s breakthrough discovery that the timescale for variation of a certain type of variable star – whether in our own Galaxy or in another – was intricately related to its intrinsic luminosity. This ‘real’ brightness is diluted as the square of the distance, so a comparison to its observed magnitude yields the distance.
Hubble’s second major finding came from plotting the distances of these spiral nebulae against the measurements of their velocities. The motion of a galaxy with respect to us is, perversely perhaps, a much more straightforward observation than determination of its distance away from us, as it can be measured from the redshift of its light in its spectrum. Even by 1917, the astronomer Vesto Slipher had already noticed that the spiral nebulae had a systematic preference for a redshift over a blueshift. Hubble confirmed that the velocities of the galaxies followed this pattern, with nearly all of them receding from us; certainly they were not all moving in the random directions would expect from a static uniform universe. Most importantly, Hubble discovered a direct proportionality between the distances to the galaxies and their redshift. This implies that the further away a galaxy is from us, the faster it is receding – a galaxy twice as far away is receding from us twice as fast. (Be aware that recent historical research indicates that the full history behind the discovery of the expansion law is more complicated than suggested in this brief précis, and also found in many popular accounts - in particular there are questions whether Hubble should really receive the sole credit.)
Expansion of the Universe
This relationship, or ‘Hubble’s Law’ as it became known, was the first evidence that space is expanding. There is an important distinction to be made; the galaxies are moving apart from each other not because we are observing the motion of galaxies through space, but because that they are pinned to a spacetime that is itself expanding. The amount of space between widely separated galaxies increases with time. This naturally explains how any light travelling towards us from a very distant object becomes redshifted: while on its journey towards us, it experiences the expansion of the space it is travelling through, which in turn stretches the wavelength of the photon. The longer the photon’s journey, the more its wavelength will have been stretched, and the more it will be redshifted.
A Creation Point?
The motion of the galaxies revealed that the Universe must have been expanding for billions of years; thus that in the past, the Universe must have been smaller and denser than it is today. Extrapolating this sufficiently back in time, the inevitable conclusion is that there must have been a time when everything in the Universe was coincident at one place, piled into an unimaginably high density of matter. It is natural to think of this as the starting point of the Universe, an idea later encapsulated as the idea of the ‘Big Bang’. The significance of Hubble’s discovery is thus not just that the Universe is expanding, but that it has a beginning… and an age. If we reverse the observed rate of the expansion of space, we can then obtain an estimate of how long it would take the Universe to expand away from the initial singularity to its current manifestation.
The Hubble Constant
To understand this, a very simple first approximation is to consider a galaxy observed at a distance d away from us; if it has been receding from us continually at its observed velocity v, it will have taken a time d/v to cover that distance. Obviously, it is more reliable to derive the time d/v for as large a sample of observed galaxies as possible; in terms of Hubble’s plot, it is measured as the reciprocal of the slope (ie gradient) of the relationship between v and d. This quantity is known as Hubble’s constant, and is measured in the esoteric astronomical units of km s-1 per Mpc (1 Mpc is a distance of 3.26 million light years). A value of the Hubble constant of 70 km s-1 Mpc-1 implies that for every 3.3 million light-years you move further out into the Universe, that bit of the Universe is moving away from us at a speed that is 70 km s-1 greater. Another way of putting it is that currently all the distances in the Universe stretch by 0.007% about every million years. The age of the Universe is also referred to as a Hubble Time. By determining the Hubble law plot for as many galaxies as far away as possible, better measurements of the Hubble constant can yield better estimates for the age of the Universe.
Hubble’s Estimate of the Hubble Constant
Although this sounds a simple idea, it didn’t seem to work well in practice at first. Hubble’s own estimate for the age of the Universe was only around 2 billion years, which was at the time (1929) significantly younger than the calculated age of the Earth from radiometric dating. This mismatch led to some initial reluctance of scientists to embrace the idea of the expanding Universe. Hubble underestimated by such a large factor due to systematic errors in his observations that led him to assume the galaxies he observed were much closer than later observations found them to be. (His data weren’t perfect, and there was confusion about different kinds of variable stars which were used for calibrating distances, and confusion of bright stars with patches of bright gaseous nebulae in distant galaxies.) Another key point is that at the time, Hubble could only establish reliable distances for the nearest, brightest galaxies; these are the ones that are receding most slowly from us, and whose motions are also most strongly affected by a local gravitational pull towards the Milky Way. The signal of the outward expansion is thus confused for our near neighbours – indeed, the Andromeda galaxy is even approaching us.
Most observational cosmology of the 20th Century was undertaken in the pursuit of refining the Hubble constant by attempting to determine the distance to systematically further galaxies. These should show the much larger velocities of recession, thus diminishing the influence of any local gravitational motions on the results. Continual correction for these errors has led to a lowering of the value of the Hubble constant, and hence a lengthening of our estimate of the Hubble time. However, even in the 1980’s we were uncertain of the value of the Hubble constant – and thus the age of the Universe! – to within a factor of two. Further embarrassment also ensued when these estimates were particularly difficult to reconcile with other, independent estimates for the age of the Universe…
Other observational limits to the age of the Universe
… which stem from the fairly obvious statement that the Universe has to be at least as old as the oldest thing within it.
Temperature of the Oldest White Dwarfs.
White dwarfs are the end-points of stellar evolution for stars similar in mass to our Sun, and are the compact core that remains after the outer layers of the late star have been blown away. A newly formed white dwarf is no longer powered by nuclear fusion reactions, and is heated initially by the process of gravitational collapse (similar to Lord Kelvin’s original idea for the power source of our Sun). Very hot when initially formed, it will radiate away this energy to cool with time; the coldest white dwarfs will be the oldest.
Unfortunately the diminishing supply of heat means that they’ll also be the faintest – and thus the hardest to actually find! Though there have been systematic searches for very faint white dwarfs though our Galaxy, the coldest so far found are still at temperatures of a few thousand degrees. As yet their estimated lifetimes are not in contradiction with the Hubble Time.
The Oldest Stars in Globular Clusters
A better way to derive a lower limit for the age of the Universe is to estimate the ages of the oldest star clusters that can be found in our galaxy. These are the globular clusters, spherical condensations of millions of stars that are distributed around the centre of our Milky Way in a halo. The combined spectra of their stars display a lack of heavy elements, suggesting they formed from near-primordial material very early on in the history of the Universe, and long before the Sun. All the stars in one of these cluster are thought to have formed at the same time from the same initial gas cloud,and thus all be the same age and of the same composition.
According to our understanding of stellar evolution, how bright and how hot a star shines depends only on its mass and its age. Stars spend most of their lives fusing hydrogen to helium in their cores, astage of their evolution known as the ‘main sequence’. During this phase, their luminosity is strongly dependent on their mass. As all the stars in a cluster are the same distance from us, the differences in their comparative brightness are solely due to the differences in their mass. Thus by plotting the range of brightnesses of the stars in a cluster, we deduce the range of stellar masses present.
But a star’s mass determines its age, in that the more massive the star, the more rapidly it exhausts its fuel. The age of the lowest-mass stars observed to still be in the main sequence phase will then set a lower limit to the age of the cluster. For example, if there are no stars more massive than 10 solar masses, then the cluster must have existed long enough for all the more massive stars to have evolved away to the next phase of their lives, and so must be at least 20 million years old. If the most massive stars in a cluster are about half the mass of the Sun, the age of the cluster would be a thousand times longer.
More recent estimates of the Hubble constant
The first reliable measurement of the Hubble constant that wasn’t far from today’s accepted was that determined in 1958 by astronomer Allan Sandage. But even then, the Hubble Time it implied made the Universe younger than the oldest known globular star clusters, estimated at that time to be 25 billion years old. Agreement between the two only came later. Partly this arose from cosmologists obtaining better measurements for ever more distant galaxies, but it was also due to improved theoretical models of stellar evolution necessary for estimating the stellar ages. A better determination of the distances to the clusters used for the age measurements also made a huge difference, showing that the stars were more luminous, and thus younger, than previously thought.
An accurate value for Hubble constant was finally determined in 2001 by Wendy Freedman and her collaborators, yielding the currently accepted rate of expansion as 73 km/s per Mpc. This no longer provided the immediate answer for the age of the Universe − cosmology had moved on in the meanwhile, however, and had been turned radically on its head during the previous decade. It had become apparent that the Hubble constant alone is not sufficient to estimate the age of the Universe – a correction is required to account for all the matter and energy content of the Universe and its role in determining the geometry of space.
The Need for a Different Cosmology
The shape of the Universe.
Everything in the Universe takes the form of either energy or matter, related to each other through the equivalence principle E=mc 2. Einstein’s theory of General Relativity expresses gravity as a curvature of space, and the mass-energy equivalence tells us that either matter or energy can produce this curvature. To quote John Wheeler “Matter tells space how to curve, and space tells matter how to move”.This is true not just locally, but also on the largest scale. Hence we can expect that the cumulative effect of all the matter and energy distributed across space gives the Universe an overall curvature. If we can measure the total curvature of space, this informs us as to how much ‘stuff’ there is in the Universe, which we express as the average density of all forms of matter and energy combined. There are several inferences for the curvature of the Universe dependent on the value of this combined mass-energy density, which we express as relative to a ‘critical’ value of this density. The critical density is that which gives a ‘flat’ shape to the Universe – where ‘flat’ is in the sense that two beams of light will remain parallel to each other even after travelling billions of light-years across the cosmos. The critical density is not very much, equivalent to only about 6 hydrogen atoms per cubic metre! A density greater than the critical density means there is more ‘stuff’ in the Universe, which exerts more gravity to pull it back and make it ‘closed’. This is described as a positive curvature, best thought of as similar to the geometry on a spherical surface: our two parallel light beams would begin to converge after a long journey, and if you follow a straight line in any direction, you would return (eventually!) to your starting position. Alternatively, a density less than this critical density describes less ‘stuff’ and gravity in the Universe, leaving it ‘open’ with negative curvature; think of this as analogous to a saddle-shaped geometry within which our two beams of parallel light eventually diverge, and any journey along a straight line would never return to the starting point.
How do we find out the shape of the Universe?
Surprisingly, the effect of gravity’s curvature on parallel light beams gives us a practical way to actually determine the curvature of the Universe; we can measure whether light rays bend towards or away from each other when they travel over a large distance. The longer the distance travelled, the greater any curvature will be, so the ideal light to study is that which has journeyed the furthest, from the Cosmic Microwave Background (CMB). The light of the CMB is not completely uniform, but it is blotched in miniscule variations, literally ‘hotspots’ in its temperature due to fluctuations in the density of the material making up the very early Universe. We know enough physics about the compression of plasmas at different temperatures to be able to calculate how big these variations should be, and so can compare the expected diameter of these fluctuations to their apparent size. Discrepancies between the two determine the curvature of the universe in between. If the Universe is closed, bending of light rays will make the spot appear smaller than it should be; if the universe is open, the light rays will bend the other way and the hotspots will appear larger than expected. Only in a flat universe will the light rays travel along straight lines so that the hotspots appear at the expected size.
The problem presented by a flat Universe
There is a clear match between the calculated and observed dimension of the hotspots in the CMB, showing that the universe is flat, ie with a mass density close to the critical density. This does, however, present a major dilemma in our understanding, for if you add up all the matter – including both the ordinary ‘baryonic’ and the dark matter – and all the radiation energy everywhere throughout the Universe, there is not enough of it. Indeed, it only sums to 27% of what is needed to fill the Universe at the critical density.
Confirmation from Clusters of Galaxies
Such a dramatic conclusion requires confirmation, preferably by a completely alternative observational technique, underpinned by different assumptions and physics used. This has been provided by X-ray observations of distant clusters of galaxies. These are the largest bound structures known, and as such can be thought of as discrete but representative samples of the Universe at different epochs, and in particular, samples of its matter content. Clusters contain mass in the form of both ordinary (mostly the X-ray hot gas) and dark matter. The ratio of the ordinary to the total matter (known as the baryonic fraction) can be measured for a cluster of galaxies (see my ‘Clusters of galaxies’ lecture for details of how we can do this!). A simple conservation principle implies that this ratio is expected to have been fixed in the very early Universe, and not change subsequently. All calculations of the baryonic fraction in samples of clusters require the adoption of a reference cosmology to turn observational quantities into absolute values (eg conversion of an X-ray flux into a luminosity depends on the distance, which in turn is dependent on the cosmology assumed…). The only cosmology that keeps the cluster baryon fraction unchanged with time is again one with only 27% of the total matter-energy content required by a flat Universe.
What distant supernovae reveal…
During the 1990’s other observational cosmologists were concentrating on establishing whether the rate of expansion itself changes with time, as another way of determining the contents of the Universe. A denser universe exerts a stronger gravitational pull that should slow down the expansion with time, whereas a less dense universe would continue expanding away to infinity. Any such changes in the rate of expansion with time would be revealed as departures in the Hubble law. When we look at galaxies within about a billion light-years from Earth, the Hubble law plot is straight, suggesting that the rate of expansion has been relatively constant over the past billion years. But if we observe much more distant galaxies, we are sampling conditions in a younger Universe; if at that time the rate of expansion of the Universe was slower or faster than it is now, then the slope of the Hubble law plot will differ and curve away from the straight line.
It is still comparatively easy to determine the motions of galaxies several billion light-years away as given by their redshift. We can, however, no longer use Leavitt’s variable star method to estimate their distances; another method is required. Two independent teams of astronomers hit on the idea of using the brightness of supernovae in very distant galaxies as distance indicators: the Supernova Cosmology Project led by Saul Perlmutter, and the High-redshift Supernova Search team led by Brian Schmidt. They don’t use just any supernovae, but only the ‘type 1a’ events. Type 1a supernovae occur only in the particular environment of a close stellar binary. Of the two stars, one has already exploded as an ordinary nova, shedding its outer layers to develop into a white dwarf. If this compact remnant remains sufficiently close to its companion, it can accrete matter, thereby growing gradually in mass. A trigger point is reached when the mass is higher than 1.4 Solar masses, as the white dwarf can no longer resist the pull of gravity with just the electron degeneracy pressure it has maintained so far, and it will re-ignite in a supernova explosion. The outburst of luminosity is so bright that it will outshine the supernova’s host galaxy to be seen half-way across the Universe, gradually fading in brightness over the next few days and weeks.
The fact that this event is triggered at a specific mass lends a certain uniformity to the explosion, including a simple observationally-derived relationship between the peak luminosity of such a supernova, and the rate at which this luminosity declines after the initial outburst: the more luminous the supernova in outburst, the slower its subsequent decrease in brightness. The observed brightness can again be related to the inferred intrinsic luminosity through the inverse square of the distance to the host galaxy.
An Accelerating Universe!
In 1998 both research groups announced the resulting Hubble Law tracing the expansion of space to much earlier epochs than had been previously sampled. The surprise was that the observed supernovae were all fainter and thus further away than they should be if one assumed their host galaxy had continued moving away at the same rate as when the light we observe left them. This is best explained if their velocity of recession has increased in the meanwhile – ie the expansion of the Universe has speeded up. We live in an accelerating Universe, and this accelerated expansion kicked in only when the Universe was about half its current age, about six billion years ago. Suddenly, a radically different view of our Universe was needed.
Introducing the idea of dark energy
There are two major observational results that have to be reconciled within cosmology:
the missing 73% of the Universe required to make it flat, which can only be in some form of energy that we cannot detect;
and the source of a ‘push’ that drives the accelerated expansion of the Universe.
We account for both by surmising that the Universe is suffused with something that we term ‘dark energy’. If it dominates our current Universe it can account for the ‘missing’ matter-energy density required to flatten the Universe. Furthermore, if dark energy provides a repellent force that has become stronger as the Universe ages, this can accelerate the expansion of space.
The Universe will have been smaller for the first half of its history, and so the attractive pull of gravity from the ordinary and dark matter will have dominated. As the Universe expands the average density of matter and radiation thins out, reducing the strength of gravitational attraction as the outward push from the dark energy becomes increasingly important. The conclusion we draw is thatunlike gravity, which is associated with matter, dark energy is instead associated with space itself - the more space you have, the stronger the push of the dark energy it generates.
What is the Dark Energy?
Do not be misled by the name of ‘dark energy’, and in particular do not confuse it with dark matter, as it is something completely different. We have simply given a name to something we don’t understand. ‘Energy’ is needed to provide the accelerated expansion, and it’s only ‘dark’ because it’s not itself observable, only the effect it has produces observable results. What the dark energy is, however, is very far from understood. The most common themes of ideas seeking to account for the dark energy are:
Einstein’s Cosmological Constant
Many scientists argue that the first intimation of dark energy was hinted at by Einstein’s inclusion of the cosmological constant, introduced to his equations as a form of repulsive ‘anti-gravity’. The appeal of this explanation is that if we could just modify the General Theory of Relativity, we could avoid theories that invoke new fields and physics. If this explanation is correct, it will change our understanding of how gravity behaves on the largest scales. However, so far there are no successful modifications of Einstein’s theory without violation of existing experimental constraints; only the addition and influence of unseen spatial dimensions may offer a graceful way out of these dilemmas.
The Vacuum Energy of Empty Space
An attractive alternative is to look for an explanation arising from the theory of particle/quantum physics, which would suggest that the dark energy is an inherent property of spacetime itself. ‘Empty space’ is a dynamic place, where temporary (and virtual) pairs of particles and antiparticles continually pop in and out of existence, created and destroyed before they can be observed. The more massive the particle involved, the shorter the existence of this ‘quantum fluctuation’. Each annihilation produces a tiny outwards pressure; the net accumulation of all these fluctuations over a vast volume of space forms an outwards push we term the ‘vacuum energy’ of space. Our current understanding of particle and quantum physics overpredicts the strength of this vacuum energy compared to what is required by astronomers to accelerate the Universe − overestimating the effect by a factor of 10120 compared to what is required by the observations! Clearly any reconciliation awaits a more complete theory of particle physics.
A new force/energy field
Of course, the dark energy could take the form of a completely new force field, sometimes described as ‘quintessence’, and which would necessarily encompass new realms of physics. This force field could become important at different epochs in the Universe’s history. Thus it could be that our current period of accelerated expansion is a milder, but related version of what inflated the early Universe during those first few fractions of a second after the Big Bang.
How do we know which is true?
Different theories seeking to account for the origin of dark energy make divergent predictions about the future expansion of the Universe. The main discriminant will be how the strength of dark energy itself evolves with time − whether it becomes diluted, remains constant, or grows stronger as the Universe increases in size. Cosmologists are aiming to carry out more detailed observations of the CMB, and seek to measure ever more distant supernovae and clusters of galaxies, all with the aim of determining the jerk of the Universe, which is the rate of change of acceleration. If the dark energy remains constant, the expansion of the universe cannot be stopped and will continue accelerating away until everything is so separated that we end in a ‘Big Chill’; alternatively if the dark energy grows strongly as the volumes of empty space increase in size, the acceleration of the expanding Universe will itself accelerate, with even more dramatic consequences encapsulated as the ‘Big Rip’. Only time will tell…
So, how old is the Universe?
Our understanding of the age of the Earth, the Sun, and the whole Universe has increased unimaginably over the last couple of centuries. If anything, the most important thing we have learnt is that there is still very much more we have yet to understand about the Universe as a whole, and that this comprehension will only introduce further exciting developments in physics.
So this may not be the ‘final’ answer, but it’s currently the best estimate we have for the age of the Universe, which I am taking from a paper by Bennett et al published in late January 2013, where they combine data from the CMB with accurate measurements of the Hubble constant. But as far we know, the age of the Universe is
13.772 ± 0.059billion years
… and of course, growing older every day.
© Professor Carolin Crawford 2013