Saul Perlmutter, who's affiliated with Lawrence Berkeley National Laboratory and the University of California, Berkeley, led one of the two independent—and rival—teams that made the discovery of the universe's increasing rate of expansion. Half the prize went to Perlmutter's team. Schmidt of the Australian National University in Canberra led the other team, which includes Riess of the Johns Hopkins University in Baltimore, Maryland. Riess and Schmidt will share the other half.
Although the accelerating cosmic expansion came as a surprise to astronomers, its discovery provided an essential ingredient in what has become the prevailing cosmological model. Thanks to dark energy—as the source of the acceleration is loosely labeled—the dynamics of our expanding universe can be reconciled with its modest mass content.
Dark energy's true nature remains a mystery. Theoretical attempts to account for its observed properties have foundered. Its existence, however, appears secure. The new Nobel laureates and their collaborators discovered the accelerated expansion by observing a certain type of reliably uniform supernova. Supporting evidence has since come from three independent sources: large-scale structure, clusters of galaxies
, and the cosmic microwave background.
The universe looks roughly the same in all directions—even parts that seem to be separated by more than the distance light has traveled since the Big Bang. To account for that puzzling uniformity, astronomers in the 1980s invoked 'inflation': a period of exponentially rapid expansion in which a tiny pocket of post–Big Bang universe spontaneously blew up to cosmic scales. Quantum fluctuations inside the pockets were amplified along with the universe. From those density fluctuations galaxies eventually formed.
Inflation's repercussions were imprinted on the expansion history of the universe. In principle, one can trace that history back in cosmic time by observing a so-called standard candle, a class of astronomical object whose intrinsic luminosity is uniform. The object's apparent brightness yields the distance; its redshift yields the velocity.
To be useful in probing expansion, a standard candle has to be both luminous (so that it can be seen at great distances and therefore long look-back times) and highly uniform (so that intrinsic scatter doesn't blur the signal). By the late 1980s, a suitable standard candle had been indentified: type Ia supernovae.
Type Ia supernovae are so luminous that they outshine their host galaxies for several weeks. They're also remarkably uniform, thanks to the special circumstances that engender them. Type Ia supernovae occur in binary systems when one of the stars, a white dwarf, accretes just enough material from its companion to tip it over the Chandrasekhar limit, the maximum mass a white dwarf can have before it collapses under its own gravity.
The chance of observing a type Ia supernova in any given galaxy is low. Only one or two are expected to pop off per galaxy per millennium. But if you could monitor the whole night sky, you'd see a type Ia supernova every few seconds.
Creating a distance–redshift diagram from type Ia supernovae requires three main steps: 1) finding the supernovae in the first place; 2) proving that they are indeed type Ia; and 3) measuring their redshifts. The first step can be accomplished at a 4-meter telescope equipped with a wide-field camera. The second and third steps require follow-up observations at an 8-meter or larger telescope or with the Hubble Space Telescope
Perlmutter and his Berkeley collaborators built a CCD-based wide-field camera to use and test at the Australian Astromical Observatory's 3.9-meter telescope, the AAT, which was built in 1976. Obtaining observing time on one of the newer, more powerful 8-meter telescopes or on the HST
was, and remains, more challenging.
To convince time-allocation panels that follow-up observations would pay off, Perlmutter devised an ingenious observing strategy. He and his collaborators would observe at Kitt Peak or other 4-m telescopes just after a new moon, comb through their data to identify new supernovae, and then apply for followup observing time just before the next new moon. The moon-free nights guaranteed that Perlmutter would find candidates at the AAT. The three-week delay guaranteed that he could confirm and characterize the candidates at the 10-m telescope at the Keck Observatory on Mauna Kea, the HST
, or another powerful telescope.
By the early 1990s, Perlmutter had acquired a large team of collaborators, called the Supernova Cosmology Project (SCP). In 1994, Schmidt formed a rival team, the High-z
Supernova Search Team (HZT), which used methods similar to those developed by SCP. Riess led the analysis of the HZT data.
The first results from HZT (10 supernovae) and SCP (42 supernovae) were published, respectively, in the Astronomical Journal
in 1998 and in the Astrophysical Journal
in 1999. Both papers reached the same conclusion: Distant supernovae are receding at a slower rate than you'd expect if matter (dark and nondark) was the only source of gravitational action. Rather than gradually slowing down, the expansion was proceeding as if it were being given an extra kick by space itself, by something that constitutes 75% of the mass–energy of the universe.
Theorists, including Albert Einstein, had been analyzing the dynamics of an expanding universe even before Edwin Hubble observed it in 1929. The SCP and HZT results are consistent with a nonzero value of the cosmological constant, Λ. Einstein had introduced Λ into his general relativity to preserve what he presumed to be the static nature of the universe. Ironically, it appears that Λ represents a substance that underlies the dynamic nature of the universe.
What could that substance be? One possibility, the vacuum energy that pervades empty space, is ruled out—at least in the form that is responsible for the Casimir force or the Lamb shift. It's too big by several orders of magnitude. Another possibility is that the accelerated expansion arises not from a substance but from a modified form of gravity, one that behaves differently on cosmic scales of time and space.
Whether dark energy has retained its value throughout the history of the universe is an important clue to its nature. Evidently, dark energy can't have been much stronger than it is today lest it prevent matter from coalescing to form galaxies. The latest observations
, which reach a redshift of about 2, are consistent with dark energy having been around in more or less its current form for the past 10 billion years.
If dark energy remains constant or if it increases in strength, then, as the cosmic expansion further reduces the density of matter and with it matter's ability to ****** the expansion, the universe will expand forever without the possibility of rebirth.
Despite the uncertainty surrounding the nature of dark energy, its discovery has increased confidence in the so-called ΛCDM model, which provides a broad framework for the evolution of the universe since the Big Bang. Features of the universe—such as the synthesis of light elements, the structure of the cosmic microwave background, and the formation of large-scale structure—all fit within the model, which assumes that Λ is the dark energy and that dark matter is cold (nonrelativistic).
With each passing moment in the Universe, we’re constantly stepping forward in time. Each successive instant gives way to the next, with time continuously appearing to flow in the same direction — forward — without fail. And yet, it isn’t particularly clear exactly why this is the case. Still, if we look for it, we can find that a number of things also happen to always move in the same direction, from moment-to-moment, exactly the way time does. Objects move through the Universe proportional to their velocity. They change their motion due to the effects of gravity and the other forces. On large scales, the Universe expands. And everywhere we look, the entropy of the Universe always goes up.
As the story of our cosmic evolution continues, we think all of these things will continue: the laws of physics will still apply just as they do today, dark energy’s presence ensures that the Universe will keep on expanding, and entropy will keep increasing, as dictated by the laws of thermodynamics. Many have speculated — although there is no proof — that the arrow of thermodynamics and the arrow of time may be related. Still others have speculated that dark energy might evolve over time, rather than being a constant, leaving the door open to the possibility that it might someday counteract and reverse our Universe’s expansion. What happens, then, if we put these speculations together?
We’d wind up imagining that perhaps the Universe will cease expanding, that it will instead begin collapsing, and that we’d have to then ask whether this means that entropy could decrease and/or time could even start running backward? It’s a mind-bending possibility, and one for the laws of physics to answer. Let’s see what they have to say about it all!
One of the most important symmetries in all of physics is known as time-reversal symmetry. Put simply, it says that the laws of physics obey the same rules whether you run the clock forward or backward. There are many examples where one phenomenon, if you run the clock forward, corresponds to an equally valid phenomenon if you run the clock backward. For example:
- A purely elastic collision, like two billiard balls colliding, would behave exactly the same if your ran the clock forward and backward, right down to the speed and angle that the balls will go off at.
- A purely inelastic collision, where two objects smash into each other and stick together, is exactly the same as a purely inelastic explosion in reverse, where the energy absorbed or released by the materials is identical.
- Gravitational interactions work the same forward and backward.
- Electromagnetic interactions behave identically forward and backward in time.
- Even the strong nuclear force, which binds atomic nuclei together, is identical forward and backward in time.
The lone exception, and the only known time where that symmetry is violated, occurs in the weak nuclear interaction: the force responsible for radioactive decays. If we ignore that outlier, the laws of physics truly are the same regardless of whether time goes forward or backward.
Individual protons and neutrons may be colorless entities, but the quarks within them are colored. Gluons can not only be exchanged between the individual gluons within a proton or neutron, but in combinations between protons and neutrons, leading to nuclear binding. However, every single exchange must obey the full suite of quantum rules.(Credit: Manishearth/Wikimedia Commons)
What this means is that, if you wind up at any final state at any moment in time, there’s always a way to get back to your initial state if you just apply the right series of interactions in just the right order. The only exception is that, if your system is complex enough, you’d have to know things like the precise positions and momenta of your particle to a better accuracy than is quantum mechanically possible
. Leaving the weak interactions and this subtle quantum rule aside, the laws of nature really are time-reversal invariant.
But this doesn’t appear to be the case for everything we experience. Some phenomena clearly display an arrow of time, or a preference for a particular one-way direction. If you grab an egg, break it, scramble it, and cook it, that’s easy; you’ll never uncook, unscramble, and un-break an egg, though, no matter how many times you try. If you push a glass off the shelf and watch it shatter against the floor, you’ll never see those bits of glass rise up and spontaneously reassemble themselves. For these examples, there clearly is a preferred direction to things: an arrow in which things flow.
A wine glass, when vibrated at the right frequency, will shatter. This is a process that dramatically increases the entropy of the system, and is thermodynamically favorable. The reverse process, of shards of glass reassembling themselves into a whole, uncracked glass, is so unlikely that it never occurs spontaneously in practice. However, wherever enough free, usable energy is present, disordered systems can become ordered, but only at the expense of increasing the overall entropy of the total system(s) in contact with one another. (Credit: BBC Worldwide/GIPHY)
Admittedly, these are complex, macroscopic systems, experiencing an extremely intricate set of interactions. Nevertheless, the combination of all these interactions adds up to something important: what we know as the thermodynamic arrow of time
. The laws of thermodynamics basically state that there are a finite number of ways that the particles in your system can be arranged, and the one(s) that have the maximum number of possible configurations — the one(s) in what we call thermodynamic equilibrium — are the ones that all systems will tend toward as time goes forward.
Your entropy, which is a measure of how statistically likely or unlikely a particular configuration is (most likely = highest entropy; very unlikely = low entropy), always rises over time. Only if you’re already in the most likely, highest entropy configuration already will your entropy stay the same over time; in any other state, your entropy will increase.
My favorite example is to imagine a room with a divider down the middle: with one side full of hot gas particles and the other full of cold gas particles. If you remove the divider, the two sides will mix and achieve the same temperature everywhere. The time-reversed situation, where you take a room of even temperature and stick a divider in the middle, spontaneously getting a hot side and a cold side, is so statistically unlikely that, given the finite age of the Universe, it never occurs.
A system set up in the initial conditions on the left and allowed to evolve will have less entropy if the door remains closed than if the door is opened. If the particles are allowed to mix, there are more ways to arrange twice as many particles at the same equilibrium temperature than there are to arrange half of those particles, each, at two different temperatures.
(Credit: Htkym & Dhollm/Wikimedia Commons)
But what could
occur, if you were willing to manipulate these particles intricately enough, is you could pump enough energy into the system to separate the particles into hot and cold, relegating one side to containing all the hot particles and the other into containing all the cold ones. This idea was put forth some 150 years ago, and goes all the way back to the person who unified electricity and magnetism into what we now know as electromagnetism: James Clerk Maxwell. It’s known, in common parlance, as Maxwell’s demon.
Imagine that you have this room full of hot-and-cold particles, and there is a central divider, but the particles are evenly distributed on both sides. Only, there’s a demon controlling the divider. Whenever a hot particle is going to smash against the divider from the “cold” side, the demon opens a gate, letting the hot particle through. Similarly, the demon also lets cold particles get through from the “hot” side. The demon has to put energy into the system to make this happen, and if you consider the demon to be part of the box/divider system, the total entropy still goes up. However, for the box/divider alone, if you were to ignore the demon, you’d see the entropy of just that box/divider system go down.
A representation of Maxwell’s demon, which can sort particles according to their energy on either side of a box. By opening and closing the divider between the two sides, the flow of particles can be intricately controlled, reducing the entropy of the system inside the box. However, the demon must exert energy to make this happen, and the overall entropy of the box+demon system still increases. (Credit: Htkym/Wikimedia Commons)
In other words, by manipulating the system appropriately from the outside, which always involves pumping energy from outside the system into the system itself, you can cause the entropy of this non-isolated system to artificially decrease.
The big question, before we even get to the Universe, is to imagine that along with these hot and cold particles, there’s also a clock inside the system. If you were inside the system, had no knowledge of the demon, but saw the gate opening and closing rapidly in various places — seemingly at random — and experienced one side of the room getting hotter while the other got colder, what would you conclude?
Would it appear that time was running backward? Would the hands on your watch start ticking backward instead of forward? Would it appear to you that the flow of time had reversed itself?
We’ve never performed this experiment, but as far as we can tell, the answer ought to be “no.” We have experienced conditions where entropy:
- increased rapidly,
- increased slowly,
- or remained the same,
both in systems on Earth and for the Universe as a whole, and as far as we can tell, time continues to always march forward at the same rate it always does: one second per second.
A light-clock, formed by a photon bouncing between two mirrors, will define time for any observer. Although the two observers may not agree with one another on how much time is passing, a successful theory will need to account for both sets of observations while having both observers agree on the laws of physics and the fundamental constants that exist within the Universe.
(Credit: John D. Norton/University of Pittsburgh)
In other words, there is a perceived arrow of time, and there is a thermodynamic arrow of time, and they both always point in the forward direction. Is this causation? While some — notably Sean Carroll — speculate that they are linked in some fashion, we should remember that is pure speculation, and that no link has ever been uncovered or demonstrated. As far as we can tell, the thermodynamic arrow of time is a consequence of statistical mechanics
, and is a property that emerged for many-body systems. (You might need at least three.) The perceived arrow of time, however, seems largely independent of anything entropy or thermodynamics may do.
What, if anything, happens when we bring the expanding Universe into the equation?
It’s true that, for all of time since (at least) the hot Big Bang, the Universe has been expanding. It’s also true that while time is linear, passing at that constant perceived rate of one second per second, the rate at which the Universe expands is not. The Universe expanded much more quickly in the past, is expanding more slowly today, and will asymptote to a finite, positive value. This, as far as we understand it, means that distant galaxies that aren’t gravitationally bound to us will continue to recede from our perspective, faster and faster, until what remains of our Local Group is the only remaining thing we can access.
The far distant fates of the Universe offer a number of possibilities, but if dark energy is truly a constant, as the data indicates, it will continue to follow the red curve, leading to the long-term scenario frequently described on Starts With A Bang: of the eventual heat death of the Universe. If dark energy evolves with time, a Big Rip or a Big Crunch are still admissible, but we don’t have any evidence indicating that this evolution is anything more than idle speculation. The steady state model, like the perfect cosmological principle, is ruled out.
(Credit: NASA/CXC/M. Weiss)
But what if this weren’t the case? What if, as in some theoretical variants of evolving dark energy, the expansion were to continue to slow down, eventually stop altogether, and then gravity would cause the Universe to contract? It’s still a plausible scenario, although the evidence doesn’t point to it, and if it pans out, the Universe could still end in a Big Crunch in the far future.
Now, if you take an expanding Universe and apply that earlier symmetry to it — time-reversal symmetry — you’ll get a contracting Universe out of it. The reverse of expansion is contraction; if you time-reversed the expanding Universe, you’d get a contracting Universe. But within that Universe, we have to look at the things that are still happening.
Gravitation is still an attractive force, and particles that fall into (or form) a bound structure still exchange energy and momentum through elastic and inelastic collisions. The normal matter particles will still shed angular momentum and collapse. They will still undergo atomic and molecular transitions and emit light and other forms of energy. To put it bluntly, everything that makes entropy increase today will still make entropy increase in a contracting Universe.
There is a large suite of scientific evidence that supports the picture of the expanding Universe and the Big Bang, complete with dark energy. The late-time accelerated expansion doesn’t strictly conserve energy, but the presence of a new component to the Universe, known as dark energy, is required to explain what we observe. (Credit: NASA / GSFC)
So if the Universe contracts, entropy will still go up. In fact, the biggest driver of entropy in our Universe is the existence and formation of supermassive black holes. Over the history of the Universe, our entropy has increased by about 30 orders of magnitude; the supermassive black hole at the center of the Milky Way alone has more entropy than the entire Universe had just 1 second after the hot Big Bang!
Not only would time continue to run forward, as far as we know, but the instant that preceded the Big Crunch would have enormously more entropy than the Universe did at the start of the hot Big Bang. All the matter and energy, under those extreme conditions, would start to merge together as all the supermassive black holes had their event horizons begin to overlap. If there were ever a scenario where gravitational waves and quantum gravitational effects could show up on macroscopic scales, this would be it. With all the matter and energy compressed into such a tiny volume, our Universe would form a supermassive black hole whose event horizon was billions of light-years across.
From outside a black hole, all the infalling matter will emit light and is always visible, while nothing from behind the event horizon can get out. The event horizon of a rotating black hole ought to be only dependent on its mass and spin, but we have not yet figured out how (or whether) the spinning black hole has a coupling to the exterior Universe. Credit: Andrew Hamilton, JILA, University of Colorado
What’s interesting about this scenario is that clocks run differently when you’re in a strong gravitational field: where you’re at small enough distances from a large enough mass. If the Universe were to recollapse and approach a Big Crunch, we’d inevitably find ourselves approaching the edge of a black hole’s event horizon, and as we did, time would begin dilating for us: stretching our final moment out toward infinity. There would be some sort of race occurring as we fell into a black hole’s central singularity, and as all the singularities merged to lead to the ultimate demise of our Universe in a Big Crunch.
What would happen after that? Would the Universe simply wink out of existence, like a complicated knot that was suddenly manipulated in such a way that it came undone? Would it lead to the birth of a new Universe, where this Big Crunch would lead to another Big Bang? Would there be some sort of cutoff, where we’d only get so far into the crunch scenario before the Universe rebounded, giving rise to some sort of rebirth without reaching a singularity?
These are some of the frontier questions of theoretical physics, and while we don’t know the answer, one thing seems to be true in all scenarios: the entropy of the entire Universe still increases, and time always runs in the forward direction. If this turns out not to be correct, it’s because there’s something profound that remains elusive to us, still waiting to be discovered.
However, if we look at the expansion of the universe, at first it was thought that, as things are expanding while objects have mass, the mass is going to be attracted to other mass, and that should slow the expansion. Then, in the late 1990’s, you have the supernova surveys that are looking deeper into space than we’ve ever looked before, and measuring distances accurately to greater distances than we’ve ever seen before. Something really surprising came out, and that was what we’ll now use “dark energy” now to explain, and that is that the acceleration is not actually slowing down – it’s not even stopped. It’s actually getting faster, and if you look at the most distant objects, they’re actually moving away from us and the acceleration is increasing the acceleration of expansion. This is actually a huge result.
One of the ideas of trying to explain it is to use the “cosmological constant,” which is something that Einstein actually introduced to his field equations to try to keep the universe the same size. He didn’t like the idea of a universe changing, so he just kind of cooked up this term and threw it into the equations to say, alright, well if it isn’t supposed to expand or contract, if I make this little mathematical adjustment, it stays the same size.
Hubble comes along about ten years later, and is observing galaxies and measuring their red shifts and their distances, and says wait a minute – no the universe is expanding. And actually we should really credit that to Georges Lemaître, who was able to interpret Hubble’s data to come up with the idea of what we now call the Big Bang.
So, the expansion’s happening – wait, it’s getting faster and faster over time. And now the attempt is to try to understand how dark energy works. Right now, most of the evidence points to this idea that the expansion will continue in the space between galaxies. That the forces of gravity, and especially magnetism and the strong nuclear force that holds protons and neutrons together in the center of an atom, would be strong enough that dark energy is never going to be able to pull those objects apart.
Nevertheless, there’s a possibility that it doesn’t work like that. There’s actually experimental evidence right now that, although it’s not well-established, that there’s a little bit of a bias with certain experiments that dark energy may continue to get stronger over time. And, if it does so, the distances won’t matter – that any object will be pulled apart. So first, you will see all galaxies recede from each other, as space starts to grow bigger and bigger, faster and faster. Then the galaxies will start to be pulled apart. Then star systems, then planets from their stars, then stars themselves, and then other objects that would typically be held together by the much stronger forces, the electromagnetic force objects held by that will be pulled apart, and then eventually, nuclei in atoms.
So if dark energy behaves so that it gets stronger and stronger over time, it will eventually overcome everything, including the collective pull of gravity of all the mass in the universe and you’ll have a universe with nothing left. That’s the ‘Big Rip’ – if dark energy gets stronger and stronger over time, it will eventually overcome any forces of attraction, and then everything is torn apart.
The cosmological model of the Big Rip is predicated on the notion that if the universe continues to accelerate in its expansion, it will eventually reach the point where all the forces that hold it together would be overcome by dark energy. Dark energy is the rather mysterious force that is predicted to make up 68% of the energy of the observable universe. If it overwhelms gravitational, electromagnetic and weak nuclear forces, the universe would literally come apart.
A model of the Big Rip theory published in 2015
actually came up with the date when the Universe would meet its demise – about 22 billion years from now. The 2015 model was developed by professor Marcelo Disconzi of Vanderbilt University in collaboration with physics professors Thomas Kephart and Robert Scherrer.
Credit: Jeremy Teaford, Vanderbilt University
Disconzi’s hypothesis* says that a Big Rip can occur when dark energy will become stronger than gravity, reaching a point when it can rip apart single atoms. The professor’s model shows that as its expansion becomes infinite, the viscosity of the universe will be responsible for its destruction. Cosmological viscosity measures how sticky
or resistant the universe is to expanding or contracting.
If the Big Rip theory is correct, one day we could reach a moment when planets and everything on them will be torn apart. Then the atomic and molecular forces will be ripped open, electrons splitting from atoms, all the way down to the quarks and anything smaller. But until then, check out his video for more on the Big Rip:
Watch: View: https://www.youtube.com/watch?v=4OdrV7gbyhU
By piecing together an increasing number of clues, cosmologists are getting closer to understanding what the future and ultimate fate of the universe will be. And I'm afraid the news is not good. Star formation will cease and black holes will take over until they eventually evaporate into nothingness. There could well be the "Big Rip" on the horizon. But for those who don't mind waiting another 101050 years or so, things may start to look up as a number of bizarre events could take place.
But before we consider random events in the very far future, let's start with what we know about the past and the present.
The reason we can investigate the past evolution of the universe
is that, in some regards, astronomy is analogous to archaeology. Explicitly: the further we peer away from our home planet, the further back in time we see in to the universe. And when we look far back in time, we observe that galaxies are closer together than they are at present. Although only one strand of evidence among many, this observation – coupled with Einstein's theory of general relativity – means that the universe started with a Big Bang and has been expanding ever since.
Late last century, one of the most pressing issues in modern cosmology was to measure the deceleration rate of the universe. Given the amount of mass observed in the cosmos it was thought that it might be enough to cause an eventual contraction of the expansion.
Remarkably, two independent teams of scientists found the exact opposite. The universe was not slowing down in its expansion, it was accelerating. This profound discovery lead to the Nobel prize in physics in 2011. However, understanding the implications of it remains challenging.
One way to think about the accelerating universe is that there must be some kind of material (or field) that permeates the universe that exerts a negative pressure (or a repulsive gravity). We call this dark energy.
This may sound a bit far-fetched, but independent experiments have been conducted to corroborate the acceleration of the universe and the existence of dark energy. From 2006, Kevin Pimbblet was involved in the WiggleZ Dark Energy Survey
– a scientific experiment to independently confirm the acceleration. Not only did we find that the acceleration is happening, but we provided compelling evidence that the cause of this was dark energy. He observed that dark energy was retarding the growth of massive superclusters of galaxies.
It is therefore suggested that dark energy is real
. If the concept of dark energy and its repulsive gravitation force is too weird, then an alternative to consider is that perhaps our theory of gravitation needs to be modified. This might be achieved in in a similar way that relativity advanced Newtonian gravitation. Either way, we need new physics to explain it.
The stronger and faster the repulsive force of dark energy is, the more likely it is that the universe will experience a Big Rip
. Put bluntly: the Big Rip is what happens when the repulsive force of dark energy
is able to overcome gravitation (and everything else). Bodies that are gravitationally bound (such as our local supercluster, our own Milky Way galaxy, our solar system, and eventually ourselves) become ripped apart
and all that is left is (probably) lonesome patches of vacuum.
The data from the WiggleZ survey and other experiments do not rule out the Big Rip, but push it in to the exceptionally far future (if at all).
Somewhat more pressing is the heat death of the universe. As the universe carries on expanding, we will no longer be able to observe galaxies outside our local group (100 million years from now). Star formation will then cease in about 1-100 trillion years
as the supply of gas needed will be exhausted. While there will be some stars around, these will run out of fuel in some 120 trillion years. All that is left at that point is stellar remnants: black holes
, neutron stars, white dwarfs being the prime examples. One hundred quintillion (1020) years from now, most of these objects will be swallowed up by the supermassive black holes
at the heart of galaxies.
In this way, the universe will get darker and quieter until there's not much going on. What happens next will depend on how fast the matter in the universe decays. It is thought that protons, which make up atoms along with neutrons and electrons, spontaneously decay into subatomic particles if you just wait long enough. The time for all ordinary matter to disappear has been calculated to be 1040 years from now. Beyond this, only black holes will remain. And even they will evaporate away
after some 10100 years.
At this point, the universe will be nearly a vacuum. Particles that remain, like electrons and light particles (photons), are then very far apart due to the universe's expansion and rarely – if at all – interact. This is the true death of the universe, dubbed the "heat death".
The idea comes from the second law of thermodynamics
, which states that entropy – a measure of "disorder" or the number of ways a system can be arranged – always increases. Any system, including the universe, will eventually evolve into a state of maximum disorder – just like a sugar cube will always dissolve in a cup of tea but would take an insanely long time to randomly go back to an orderly cube structure. When all the energy the in the cosmos is uniformly spread out, there is no more heat or free energy to fuel processes that consume energy, such as life.
Turn for a moment to Ginnungagap, the Norse mythology concerning the chaos/cosmic split:
Ginnungagap is the bottomless abyss that was all there was prior to the creation of the cosmos
, and into which the cosmos will collapse once again during Ragnarok
, the “Twilight of the Gods” or, in German, "Götterdämmerung."
poem Völuspá, “The Insight of the Seeress,” describes the time before the cosmos existed:
"That was the age when nothing was;
There was no sand, nor sea, nor cool waves,
No earth nor sky nor grass there,
The Old Norse
word gap means the same thing as it does in modern English: a void, an empty space. The meaning of the ginnung element, however, is far less certain. The best guess anyone has come up with so far is Jan de Vries’s suggestion of “magically
-charged,” a theory that has gained widespread acceptance. This surely refers to the capacity for something that can serve as the basis for creation to come out of its nothingness.
This perfect, uninterrupted silence and darkness has close counterparts in other mythologies from around the world. To cite but one example, most will no doubt be familiar with the famous words of the first chapter of Genesis, which describe the state of the universe prior to the intervention of Elohim in Judeo-Christian mythology: “And the earth was without form, and void; and darkness was upon the face of the deep.” The opposition between the well-ordered, just, and beneficent cosmos on the one hand and the lawless chaos that surrounds it is perhaps one of the most common themes in religion and in human consciousness more generally.
In the pre-Christian religion of the Norse and other Germanic peoples, this chaos-cosmos split is expressed as an opposition between the innangard
, that which is orderly, civilized, and law-abiding, and the utangard
, that which is wild and anarchic. Plowed fields are innangard, but beyond the fences that surround them and mark them off reigns the wilderness, the utangard home of the giants
. These anti-cosmic forces are constantly trying to drag the Aesir
gods, their work, and their ideals back to chaos (and at Ragnarok they will succeed). While the wilderness is utangard enough, the “capital” of chaos, as it were, is Ginnungagap; the abyss is the ultimate destination to which the giants want to bring the world.
Several modern philosophers associated with existentialism, a movement that takes our experience of existence as the starting point of its philosophizing, have spoken of a similar schema using the more prosaic and impersonal language of philosophy and psychology. While the writings of luminaries such as Søren Kierkegaard, Martin Heidegger, and Jean-Paul Sartre differ considerably on these points, a fascination with negation and anxiety is a central focus of their work. In existentialist parlance, “nothingness” is that which negates oneself, one’s values, and/or one’s worldview – one’s “personal cosmos.”
The ultimate nothingness is death, because it negates one absolutely (at least in the modern worldview – see Death and the Afterlife
for Norse perspective on death), but any condition over which one cannot triumph is a hostile absence into which one’s yearnings, strivings, and beliefs vanish. This negation is the root of anxiety (or “angst” or “Being-toward-death”), the fear of what we might not be able to overcome, that which stands every chance of “getting us” in the end. This is one of the fundamental facts of life with which everyone who strives to live deliberately and authentically must grapple. In Heidegger’s words, “To be a particular being means to be immersed in nothingness.” While these philosophers don’t necessarily identify nothingness with a physical void as the Norse did, the principle remains the same.
This primordial, annihilating chaos is ever-present; wherever there is darkness, wherever there is silence, wherever any wish or belief is negated, there is Ginnungagap.
Looking for more great information on Norse mythology and religion? Examine the popular list of The 10 Best Norse Mythology Books
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Additionally, though somewhat dated, you might read The Triumph of Time by James Blish, 1958, the last book a four book series covering Cities in Flight, when cities from Earth take to outer space thanks to the discovery of an antigravity machine. It feature's Blish's view of the end of time.
What do I think? We are probably headed for the heat death scenario in the very distant future, regardless of whether or not humans can leave Earth for the stars before the sun becomes a red giant and increases the Earth, moving of the main sequence of the Hertzsprung/Russell diagram, also called H-R diagram, in astronomy
. It is a graph in which the absolute magnitudes
(intrinsic brightness) of stars are plotted against their spectral types (temperatures).
Of great importance to theories of stellar evolution, it evolved from charts begun in 1911 by the Danish astronomer Ejnar Hertzsprung
and independently by the U.S. astronomer Henry Norris Russell
Hertzsprung-Russell diagram of solar neighbourhood
On the diagram stars are ranked from bottom to top in order of decreasing magnitude (increasing brightness) and from right to left by increasing temperature (spectral class). Stars of the galactic arm in which the Sun is located tend to fall into distinct regions on the diagram. The group called the main sequence extends in a rough diagonal from the upper left of the diagram (hot, bright stars) to the lower right (dim and cool). Large, bright, though cool, stars called giants and supergiants appear in the upper right, and the white dwarfs, dim, small, and hot, lie in the lower left.
The Sun lies near the middle of the main sequence, and stars spend most of their lives on the main sequence. As stars burn up the hydrogen
in their cores into helium
, they become more luminous
and cooler (because they have expanded) and therefore move off the main sequence into the upper right region of the giants and supergiants.
The point at which stars move off the main sequence can be used to give the age of star clusters
, because stars at the lower end of the main sequence take longer to burn their hydrogen into helium than stars at the upper end. The most massive stars explode in supernovas
. Stars of a few solar masses eject their outer layers as planetary nebulae
, which have a hot, luminous central star
found in the upper left of the diagram. Stars like the Sun burn down to cool white dwarfs
, which are found in the bottom left of the diagram.
Whether or not humans have interstellar travel before the Sun expands will, sadly only prolong our species until the ultimate heat death, entropy, overcomes us all.