Tag Archives: dark energy

The Accelerated Acceleration

Dark_Energy

Until the mid-1990s accepted scientific understanding of the universe held that the cosmos was expanding. Scientists have accepted this since 1929 when Edwin Hubble‘s celestial observations showed that distant galaxies were all apparently moving away from us.

But, in 1998 two independent groups of cosmologists made a startling finding. The universe was not only expanding, its expansion was accelerating. Recent studies show that this acceleration in the fabric of spacetime is actually faster than first theorized and observed.

And, nobody knows why. This expansion, indeed the accelerating expansion, remains one of our current great scientific mysteries.

Cosmologists, astronomers and theoreticians of all stripes have proposed no shortage of possible explanations. But, there is still scant observational evidence to support any of the leading theories. The most popular revolves around the peculiar idea of dark energy.

From Scientific American:

Our universe is flying apart, with galaxies moving away from each other faster each moment than they were the moment before. Scientists have known about this acceleration since the late 1990s, but whatever is causing it—dubbed dark energy—remains a mystery. Now the latest measurement of how fast the cosmos is growing thickens the plot further: The universe appears to be ballooning more quickly than it should be, even after accounting for the accelerating expansion caused by dark energy.

Scientists came to this conclusion after comparing their new measurement of the cosmic expansion rate, called the Hubble constant, to predictions of what the Hubble constant should be based on evidence from the early universe. The puzzling conflict—which was hinted at in earlier data and confirmed in the new calculation—means that either one or both of the measurements are flawed, or that dark energy or some other aspect of nature acts differently than we think.

“The bottom line is that the universe looks like it’s expanding about eight percent faster than you would have expected based on how it looked in its youth and how we expect it to evolve,” says study leader Adam Riess of the Space Telescope Science Institute in Baltimore, Md. “We have to take this pretty darn seriously.” He and his colleagues described their findings, based on observations from the Hubble Space Telescope, in a paper submitted last week to the Astrophysical Journal and posted on the preprint server arXiv.

One of the most exciting possibilities is that dark energy is even stranger than the leading theory suggests. Most observations support the idea that dark energy behaves like a “cosmological constant,” a term Albert Einstein inserted into his equations of general relativity and later removed. This kind of dark energy would arise from empty space, which, according to quantum mechanics, is not empty at all, but rather filled with pairs of “virtual” particles and antiparticles that constantly pop in and out of existence. These virtual particles would carry energy, which in turn might exert a kind of negative gravity that pushes everything in the universe outward.

Read the entire story here.

Image: The universe’s accelerated expansion. Courtesy: NASA and ESA.

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95.5 Percent is Made Up and It’s Dark

Petrarch_by_Bargilla

Physicists and astronomers observe the very small and the very big. Although they are focused on very different areas of scientific endeavor and discovery, they tend to agree on one key observation: 95.5 of the cosmos is currently invisible to us. That is, only around 4.5 percent of our physical universe is made up of matter or energy that we can see or sense directly through experimental interaction. The rest, well, it’s all dark — so-called dark matter and dark energy. But nobody really knows what or how or why. Effectively, despite tremendous progress in our understanding of our world, we are still in a global “Dark Age”.

From the New Scientist:

TO OUR eyes, stars define the universe. To cosmologists they are just a dusting of glitter, an insignificant decoration on the true face of space. Far outweighing ordinary stars and gas are two elusive entities: dark matter and dark energy. We don’t know what they are… except that they appear to be almost everything.

These twin apparitions might be enough to give us pause, and make us wonder whether all is right with the model universe we have spent the past century so carefully constructing. And they are not the only thing. Our standard cosmology also says that space was stretched into shape just a split second after the big bang by a third dark and unknown entity called the inflaton field. That might imply the existence of a multiverse of countless other universes hidden from our view, most of them unimaginably alien – just to make models of our own universe work.

Are these weighty phantoms too great a burden for our observations to bear – a wholesale return of conjecture out of a trifling investment of fact, as Mark Twain put it?

The physical foundation of our standard cosmology is Einstein’s general theory of relativity. Einstein began with a simple observation: that any object’s gravitational mass is exactly equal to its resistance to accelerationMovie Camera, or inertial mass. From that he deduced equations that showed how space is warped by mass and motion, and how we see that bending as gravity. Apples fall to Earth because Earth’s mass bends space-time.

In a relatively low-gravity environment such as Earth, general relativity’s effects look very like those predicted by Newton’s earlier theory, which treats gravity as a force that travels instantaneously between objects. With stronger gravitational fields, however, the predictions diverge considerably. One extra prediction of general relativity is that large accelerating masses send out tiny ripples in the weave of space-time called gravitational waves. While these waves have never yet been observed directly, a pair of dense stars called pulsars, discovered in 1974, are spiralling in towards each other just as they should if they are losing energy by emitting gravitational waves.

Gravity is the dominant force of nature on cosmic scales, so general relativity is our best tool for modelling how the universe as a whole moves and behaves. But its equations are fiendishly complicated, with a frightening array of levers to pull. If you then give them a complex input, such as the details of the real universe’s messy distribution of mass and energy, they become effectively impossible to solve. To make a working cosmological model, we make simplifying assumptions.

The main assumption, called the Copernican principle, is that we are not in a special place. The cosmos should look pretty much the same everywhere – as indeed it seems to, with stuff distributed pretty evenly when we look at large enough scales. This means there’s just one number to put into Einstein’s equations: the universal density of matter.

Einstein’s own first pared-down model universe, which he filled with an inert dust of uniform density, turned up a cosmos that contracted under its own gravity. He saw that as a problem, and circumvented it by adding a new term into the equations by which empty space itself gains a constant energy density. Its gravity turns out to be repulsive, so adding the right amount of this “cosmological constant” ensured the universe neither expanded nor contracted. When observations in the 1920s showed it was actually expanding, Einstein described this move as his greatest blunder.

It was left to others to apply the equations of relativity to an expanding universe. They arrived at a model cosmos that grows from an initial point of unimaginable density, and whose expansion is gradually slowed down by matter’s gravity.

This was the birth of big bang cosmology. Back then, the main question was whether the expansion would ever come to a halt. The answer seemed to be no; there was just too little matter for gravity to rein in the fleeing galaxies. The universe would coast outwards forever.

Then the cosmic spectres began to materialise. The first emissary of darkness put a foot in the door as long ago as the 1930s, but was only fully seen in the late 1970s when astronomers found that galaxies are spinning too fast. The gravity of the visible matter would be too weak to hold these galaxies together according to general relativity, or indeed plain old Newtonian physics. Astronomers concluded that there must be a lot of invisible matter to provide extra gravitational glue.

The existence of dark matter is backed up by other lines of evidence, such as how groups of galaxies move, and the way they bend light on its way to us. It is also needed to pull things together to begin galaxy-building in the first place. Overall, there seems to be about five times as much dark matter as visible gas and stars.

Dark matter’s identity is unknown. It seems to be something beyond the standard model of particle physics, and despite our best efforts we have yet to see or create a dark matter particle on Earth (see “Trouble with physics: Smashing into a dead end”). But it changed cosmology’s standard model only slightly: its gravitational effect in general relativity is identical to that of ordinary matter, and even such an abundance of gravitating stuff is too little to halt the universe’s expansion.

The second form of darkness required a more profound change. In the 1990s, astronomers traced the expansion of the universe more precisely than ever before, using measurements of explosions called type 1a supernovae. They showed that the cosmic expansion is accelerating. It seems some repulsive force, acting throughout the universe, is now comprehensively trouncing matter’s attractive gravity.

This could be Einstein’s cosmological constant resurrected, an energy in the vacuum that generates a repulsive force, although particle physics struggles to explain why space should have the rather small implied energy density. So imaginative theorists have devised other ideas, including energy fields created by as-yet-unseen particles, and forces from beyond the visible universe or emanating from other dimensions.

Whatever it might be, dark energy seems real enough. The cosmic microwave background radiation, released when the first atoms formed just 370,000 years after the big bang, bears a faint pattern of hotter and cooler spots that reveals where the young cosmos was a little more or less dense. The typical spot sizes can be used to work out to what extent space as a whole is warped by the matter and motions within it. It appears to be almost exactly flat, meaning all these bending influences must cancel out. This, again, requires some extra, repulsive energy to balance the bending due to expansion and the gravity of matter. A similar story is told by the pattern of galaxies in space.

All of this leaves us with a precise recipe for the universe. The average density of ordinary matter in space is 0.426 yoctograms per cubic metre (a yoctogram is 10-24 grams, and 0.426 of one equates to about 250 protons), making up 4.5 per cent of the total energy density of the universe. Dark matter makes up 22.5 per cent, and dark energy 73 per cent (see diagram). Our model of a big-bang universe based on general relativity fits our observations very nicely – as long as we are happy to make 95.5 per cent of it up.

Arguably, we must invent even more than that. To explain why the universe looks so extraordinarily uniform in all directions, today’s consensus cosmology contains a third exotic element. When the universe was just 10-36 seconds old, an overwhelming force took over. Called the inflaton field, it was repulsive like dark energy, but far more powerful, causing the universe to expand explosively by a factor of more than 1025, flattening space and smoothing out any gross irregularities.

When this period of inflation ended, the inflaton field transformed into matter and radiation. Quantum fluctuations in the field became slight variations in density, which eventually became the spots in the cosmic microwave background, and today’s galaxies. Again, this fantastic story seems to fit the observational facts. And again it comes with conceptual baggage. Inflation is no trouble for general relativity – mathematically it just requires an add-on term identical to the cosmological constant. But at one time this inflaton field must have made up 100 per cent of the contents of the universe, and its origin poses as much of a puzzle as either dark matter or dark energy. What’s more, once inflation has started it proves tricky to stop: it goes on to create a further legion of universes divorced from our own. For some cosmologists, the apparent prediction of this multiverse is an urgent reason to revisit the underlying assumptions of our standard cosmology (see “Trouble with physics: Time to rethink cosmic inflation?”).

The model faces a few observational niggles, too. The big bang makes much more lithium-7 in theory than the universe contains in practice. The model does not explain the possible alignment in some features in the cosmic background radiation, or why galaxies along certain lines of sight seem biased to spin left-handedly. A newly discovered supergalactic structure 4 billion light years long calls into question the assumption that the universe is smooth on large scales.

Read the entire story here.

Image: Petrarch, who first conceived the idea of a European “Dark Age”, by Andrea di Bartolo di Bargilla, c1450. Courtesy of Galleria degli Uffizi, Florence, Italy / Wikipedia.

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Mutant Gravity and Dark Magnetism

Scientific consensus states that our universe is not only expanding, but expanding at an ever-increasing rate. So, sometime in the very distant future (tens of billions of years) our Milky Way galaxy will be mostly alone, accompanied only by its close galactic neighbors, such as Andromeda. All else in the universe will have receded beyond the horizon of visible light. And, yet for all the experimental evidence, no one knows the precise cause(s) of this acceleration or even of the expansion itself. But, there is no shortage of bold new theories.

From New Scientist:

WE WILL be lonely in the late days of the cosmos. Its glittering vastness will slowly fade as countless galaxies retreat beyond the horizon of our vision. Tens of billions of years from now, only a dense huddle of nearby galaxies will be left, gazing out into otherwise blank space.

That gloomy future comes about because space is expanding ever faster, allowing far-off regions to slip across the boundary from which light has time to reach us. We call the author of these woes dark energy, but we are no nearer to discovering its identity. Might the culprit be a repulsive force that emerges from the energy of empty spaceMovie Camera, or perhaps a modification of gravity at the largest scales? Each option has its charms, but also profound problems.

But what if that mysterious force making off with the light of the cosmos is an alien echo of light itself? Light is just an expression of the force of electromagnetism, and vast electromagnetic waves of a kind forbidden by conventional physics, with wavelengths trillions of times larger than the observable universe, might explain dark energy’s baleful presence. That is the bold notion of two cosmologists who think that such waves could also account for the mysterious magnetic fields that we see threading through even the emptiest parts of our universe. Smaller versions could be emanating from black holes within our galaxy.

It is almost two decades since we realised that the universe is running away with itself. The discovery came from observations of supernovae that were dimmer, and so further away, than was expected, and earned its discoverers the Nobel prize in physics in 2011.

Prime suspect in the dark-energy mystery is the cosmological constant, an unchanging energy which might emerge from the froth of short-lived, virtual particles that according to quantum theory are fizzing about constantly in otherwise empty space.

Mutant gravity

To cause the cosmic acceleration we see, dark energy would need to have an energy density of about half a joule per cubic kilometre of space. When physicists try to tot up the energy of all those virtual particles, however, the answer comes to either exactly zero (which is bad), or something so enormous that empty space would rip all matter to shreds (which is very bad). In this latter case the answer is a staggering 120 orders of magnitude out, making it a shoo-in for the least accurate prediction in all of physics.

This stumbling block has sent some researchers down another path. They argue that in dark energy we are seeing an entirely new side to gravity. At distances of many billions of light years, it might turn from an attractive to a repulsive force.

But it is dangerous to be so cavalier with gravity. Einstein’s general theory of relativity describes gravity as the bending of space and time, and predicts the motions of planets and spacecraft in our own solar system with cast-iron accuracy. Try bending the theory to make it fit acceleration on a cosmic scale, and it usually comes unstuck closer to home.

That hasn’t stopped many physicists persevering along this route. Until recently, Jose Beltrán and Antonio Maroto were among them. In 2008 at the Complutense University of Madrid, Spain, they were playing with a particular version of a mutant gravity model called a vector-tensor theory, which they had found could mimic dark energy. Then came a sudden realisation. The new theory was supposed to be describing a strange version of gravity, but its equations bore an uncanny resemblance to some of the mathematics underlying another force. “They looked like electromagnetism,” says Beltrán, now based at the University of Geneva in Switzerland. “We started to think there could be a connection.”

So they decided to see what would happen if their mathematics described not masses and space-time, but magnets and voltages. That meant taking a fresh look at electromagnetism. Like most of nature’s fundamental forces, electromagnetism is best understood as a phenomenon in which things come chopped into little pieces, or quanta. In this case the quanta are photons: massless, chargeless particles carrying fluctuating electric and magnetic fields that point at right angles to their direction of motion.

Alien photons

This description, called quantum electrodynamics or QED, can explain a vast range of phenomena, from the behaviour of light to the forces that bind molecules together. QED has arguably been tested more precisely than any other physical theory, but it has a dark secret. It wants to spit out not only photons, but also two other, alien entities.

The first kind is a wave in which the electric field points along the direction of motion, rather than at right angles as it does with ordinary photons. This longitudinal mode moves rather like a sound wave in air. The second kind, called a temporal mode, has no magnetic field. Instead, it is a wave of pure electric potential, or voltage. Like all quantum entities, these waves come in particle packets, forming two new kinds of photon.

As we have never actually seen either of these alien photons in reality, physicists found a way to hide them. They are spirited away using a mathematical fix called the Lorenz condition, which means that all their attributes are always equal and opposite, cancelling each other out exactly. “They are there, but you cannot see them,” says Beltrán.

Beltrán and Maroto’s theory looked like electromagnetism, but without the Lorenz condition. So they worked through their equations to see what cosmological implications that might have.

The strange waves normally banished by the Lorenz condition may come into being as brief quantum fluctuations – virtual waves in the vacuum – and then disappear again. In the early moments of the universe, however, there is thought to have been an episode of violent expansion called inflation, which was driven by very powerful repulsive gravity. The force of this expansion grabbed all kinds of quantum fluctuations and amplified them hugely. It created ripples in the density of matter, for example, which eventually seeded galaxies and other structures in the universe.

Crucially, inflation could also have boosted the new electromagnetic waves. Beltrán and Maroto found that this process would leave behind vast temporal modes: waves of electric potential with wavelengths many orders of magnitude larger than the observable universe. These waves contain some energy but because they are so vast we do not perceive them as waves at all. So their energy would be invisible, dark… perhaps, dark energy?

Beltrán and Maroto called their idea dark magnetism (arxiv.org/abs/1112.1106). Unlike the cosmological constant, it may be able to explain the actual quantity of dark energy in the universe. The energy in those temporal modes depends on the exact time inflation started. One plausible moment is about 10 trillionths of a second after the big bang, when the universe cooled below a critical temperature and electromagnetism split from the weak nuclear force to become a force in its own right. Physics would have suffered a sudden wrench, enough perhaps to provide the impetus for inflation.

If inflation did happen at this “electroweak transition”, Beltrán and Maroto calculate that it would have produced temporal modes with an energy density close to that of dark energy. The correspondence is only within an order of magnitude, which may not seem all that precise. In comparison with the cosmological constant, however, it is mildly miraculous.

The theory might also explain the mysterious existence of large-scale cosmic magnetic fields. Within galaxies we see the unmistakable mark of magnetic fields as they twist the polarisation of light. Although the turbulent formation and growth of galaxies could boost a pre-existing field, is it not clear where that seed field would have come from.

Even more strangely, magnetic fields seem to have infiltrated the emptiest deserts of the cosmos. Their influence was noticed in 2010 by Andrii Neronov and Ievgen Vovk at the Geneva Observatory. Some distant galaxies emit blistering gamma rays with energies in the teraelectronvolt range. These hugely energetic photons should smack into background starlight on their way to us, creating electrons and positrons that in turn will boost other photons up to gamma energies of around 100 gigaelectronvolts. The trouble is that astronomers see relatively little of this secondary radiation. Neronov and Vovk suggest that is because a diffuse magnetic field is randomly bending the path of electrons and positrons, making their emission more diffuse (Science, vol 32, p 73).

“It is difficult to explain cosmic magnetic fields on the largest scales by conventional mechanisms,” says astrophysicist Larry Widrow of Queen’s University in Kingston, Ontario, Canada. “Their existence in the voids might signal an exotic mechanism.” One suggestion is that giant flaws in space-time called cosmic strings are whipping them up.

With dark magnetism, such a stringy solution would be superfluous. As well as the gigantic temporal modes, dark magnetism should also lead to smaller longitudinal waves bouncing around the cosmos. These waves could generate magnetism on the largest scales and in the emptiest voids.

To begin with, Beltrán and Maroto had some qualms. “It is always dangerous to modify a well-established theory,” says Beltrán. Cosmologist Sean Carroll at the California Institute of Technology in Pasadena, echoes this concern. “They are doing extreme violence to electromagnetism. There are all sorts of dangers that things might go wrong,” he says. Such meddling could easily throw up absurdities, predicting that electromagnetic forces are different from what we actually see.

The duo soon reassured themselves, however. Although the theory means that temporal and longitudinal modes can make themselves felt, the only thing that can generate them is an ultra-strong gravitational field such as the repulsive field that sprang up in the era of inflation. So within the atom, in all our lab experiments, and out there among the planets, electromagnetism carries on in just the same way as QED predicts.

Carroll is not convinced. “It seems like a long shot,” he says. But others are being won over. Gonzalo Olmo, a cosmologist at the University of Valencia, Spain, was initially sceptical but is now keen. “The idea is fantastic. If we quantise electromagnetic fields in an expanding universe, the effect follows naturally.”

So how might we tell whether the idea is correct? Dark magnetism is not that easy to test. It is almost unchanging, and would stretch space in almost exactly the same way as a cosmological constant, so we can’t tell the two ideas apart simply by watching how cosmic acceleration has changed over time.

Ancient mark

Instead, the theory might be challenged by peering deep into the cosmic microwave background, a sea of radiation emitted when the universe was less than 400,000 years old. Imprinted on this radiation are the original ripples of matter density caused by inflation, and it may bear another ancient mark. The turmoil of inflation should have energised gravitational waves, travelling warps in space-time that stretch and squeeze everything they pass through. These waves should affect the polarisation of cosmic microwaves in a distinctive way, which could tell us about the timing and the violence of inflation. The European Space Agency’s Planck spacecraft might just spot this signature. If Planck or a future mission finds that inflation happened before the electroweak transition, at a higher energy scale, then that would rule out dark magnetism in its current form.

Olmo thinks that the theory might anyhow need some numerical tweaking, so that might not be fatal, although it would be a blow to lose the link between the electroweak transition and the correct amount of dark energy.

One day, we might even be able to see the twisted light of dark magnetism. In its present incarnation with inflation at the electroweak scale, the longitudinal waves would all have wavelengths greater than a few hundred million kilometres, longer than the distance from Earth to the sun. Detecting a light wave efficiently requires an instrument not much smaller than the wavelength, but in the distant future it might just be possible to pick up such waves using space-based radio telescopes linked up across the solar system. If inflation kicked in earlier at an even higher energy, as suggested by Olmo, some of the longitudinal waves could be much shorter. That would bring them within reach of Earth-based technology. Beltrán suggests that they might be detected with the Square Kilometre Array – a massive radio instrument due to come on stream within the next decade.

If these dark electromagnetic waves can be created by strong gravitational fields, then they could also be produced by the strongest fields in the cosmos today, those generated around black holes. Beltrán suggests that waves may be emitted by the black hole at the centre of the Milky Way. They might be short enough for us to see – but they could easily be invisibly faint. Beltrán and Maroto are planning to do the calculations to find out.

One thing they have calculated from their theory is the voltage of the universe. The voltage of the vast temporal waves of electric potential started at zero when they were first created at the time of inflation, and ramped up steadily. Today, it has reached a pretty lively 1027 volts, or a billion billion gigavolts.

Just as well for us that it has nowhere to discharge. Unless, that is, some other strange quirk of cosmology brings a parallel universe nearby. The encounter would probably destroy the universe as we know it, but at least then our otherwise dark and lonely future would end with the mother of all lightning bolts.

Read the entire article after the jump.

Graphic courtesy of NASA / WMAP.

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Dark energy spotted in the cosmic microwave background

From Institute of Physics:

Astronomers studying the cosmic microwave background (CMB) have uncovered new direct evidence for dark energy – the mysterious substance that appears to be accelerating the expansion of the universe. Their findings could also help map the structure of dark matter on the universe’s largest length scales.

The CMB is the faint afterglow of the universe’s birth in the Big Bang. Around 400,000 years after its creation, the universe had cooled sufficiently to allow electrons to bind to atomic nuclei. This “recombination” set the CMB radiation free from the dense fog of plasma that was containing it. Space telescopes such as WMAP and Planck have charted the CMB and found its presence in all parts of the sky, with a temperature of 2.7 K. However, measurements also show tiny fluctuations in this temperature on the scale of one part in a million. These fluctuations follow a Gaussian distribution.

In the first of two papers, a team of astronomers including Sudeep Das at the University of California, Berkeley, has uncovered fluctuations in the CMB that deviate from this Gaussian distribution. The deviations, observed with the Atacama Cosmology Telescope in Chile, are caused by interactions with large-scale structures in the universe, such as galaxy clusters. “On average, a CMB photon will have encountered around 50 large-scale structures before it reaches our telescope,” Das told physicsworld.com. “The gravitational influence of these structures, which are dominated by massive clumps of dark matter, will each deflect the path of the photon,” he adds. This process, called “lensing”, eventually adds up to a total deflection of around 3 arc minutes – one-20th of a degree.

Dark energy versus structure

In the second paper Das, along with Blake Sherwin of Princeton University and Joanna Dunkley of Oxford University, looks at how lensing could reveal dark energy. Dark energy acts to counter the emergence of structures within the universe. A universe with no dark energy would have a lot of structure. As a result, the CMB photons would undergo greater lensing and the fluctuations would deviate more from the original Gaussian distribution.

More from theSource here.

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The Universe’s Invisible Hand

From Scientific American:

Dark energy does more than hurry along the expansion of the universe. It also has a stranglehold on the shape and spacing of galaxies.

What took us so long? Only in 1998 did astronomers discover we had been missing nearly three quarters of the contents of the universe, the so-called dark energy–an unknown form of energy that surrounds each of us, tugging at us ever so slightly, holding the fate of the cosmos in its grip, but to which we are almost totally blind. Some researchers, to be sure, had anticipated that such energy existed, but even they will tell you that its detection ranks among the most revolutionary discoveries in 20th-century cosmology. Not only does dark energy appear to make up the bulk of the universe, but its existence, if it stands the test of time, will probably require the development of new theories of physics.

Scientists are just starting the long process of figuring out what dark energy is and what its implications are. One realization has already sunk in: although dark energy betrayed its existence through its effect on the universe as a whole, it may also shape the evolution of the universe’s inhabitants–stars, galaxies, galaxy clusters. Astronomers may have been staring at its handiwork for decades without realizing it.

More from theSource here.

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