Substance

 The Substance of our Universe

Dark Energy

It is more than everything else in this universe and it will govern the fate of the Universe. Dark energy is everywhere. It suffuses every corner of the cosmos, absolutely dominating everything in it. It dictates how the universe behaves now and how it will end. Physics has not yet explained it though we know something is out there.

Dark energy is the placeholder’s name that physicists use for the energy of space that is noticeably accelerating the expansion of the universe. We know almost nothing about it (as of the time of writing this document), but there are many hypotheses. There are telescopes and radio detectors that are mapping more and more of the Universe in the hope of finding some clues of dark energy. Space missions are being planned to examine ‘it’. Stellar explosions can be used to provide some insight into its influence on the early Universe. Gravitational waves may also have a part to play, as gravitational-wave detectors begin to listen for the effect of dark energy on the echoes of colliding black holes.

About the best that can be said about dark energy is that it acts as a kind of antigravity, pushing things apart where gravity pulls them together. Dark energy doesn’t absorb or emit light, so don’t let the name confuse you but it is something like a perpetual motion machine making up about 68% of all the universes energy. Regular matter, the stuff stars and planets are made from, makes up less than 5% with the remainder going to dark matter.

One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe.

Dark energy accounts for the accelerating expansion of the universe.

Dark Energy Estimate

Standard Candles

It is only recent that scientists have realized that the universe is accelerating though they have known since the late 1920s that the Universe is expanding. Initially they assumed that expansion should slow down because the force of gravity between everything in the universe generates an attractive force and gradually subtracts energy from the expansion and reduces the velocity, but it was in 1998 that two teams found the opposite. They were searching for stellar explosions of a particular type called the standard candle. These are found by observing type 1a supernovae, which occur when white-dwarf stars undergo a runaway nuclear reaction. The spectrum of the light and its intensity is known due to this type of super nova and this gives an excellent way to calculate the distance from the brightness and then measure the relative velocity by its redshift. The intrinsic brightness of these type 1a supernovae is fixed by how fast its light fades—brighter ones burn more briefly. To be more specific; by counting how many days a type 1a takes to fade, you can work out how much light the explosion emitted; then, by measuring its apparent brightness at Earth, you can calculate how far away the supernovae is and how long the light has been travelling. Also, as was mentioned one can measure the supernova's redshift. The redshift as we know determines the amount by which the wavelength of light has been stretched out since it was emitted, which reveals the degree by which space has expanded. The intrinsic colour (frequency spectrum) of these supernova is known so the redshift can be accurately measured. Combining these observations allows astronomers to determine the expansion of the Universe over time—that is how both teams discovered that the speed of expansion is not slowing, but accelerating. Something is acting against gravity which has been coined dark energy.

A second type of observation took place tens years later, in 2008 which supports the 1998 findings. Instead of using Type IA supernovae, this study was based on observations of clusters of galaxies at different time points in the history of the universe.

The team that did this second measurement, led by Alexey Vikhlininof the Smithsonian Astrophysical Observatory in Cambridge, Mass., used the Chandra X-ray Observatory to measure the hot gas in dozens of galaxy clusters, which are the largest collapsed objects in the universe.

Some clusters are relatively nearby and others are more than halfway across the universe giving them a scale to calibrate. They studied the X-rays emitted from the hot gas as it fell in to areas that are full of dark matter. The X-rays can be converted into mass for a given cluster at a given point in time (depending on the age of the cluster). Dark matter with its constant gravitational tug will try to slow things down and form clumps while the dark energy is trying to speed things up, eventually making it hard for the galaxies or the dark matter to cluster.

When astronomers look farther out at the cosmos and hence farther back in time, the results show an increase in the mass of the galaxy clusters further back in time, which supports the idea that dark energy started to win out in the tug of war at some point in the universe's history. Astronomers are not certain on the timing of the change from an expanding universe to one whose expansion is speeding up. Though 5 billion years is the current estimate.

All that is known so far about this force is that it is pushing outward and is greater than that of gravity. What they are seeking now is if its strength has changed across time. These type 1A supernova can help with this quest.

During the first parts of the expansion of the universe, before matter had really crystalized out of the hot soup, it is expected that sound waves were bouncing around inside the stuff the universe was made of at that time. After a while, about 400,000 years, the universe cooled enough for the ions to capture electrons and form into matter. When this happened the sonic waves stopped as their medium basically froze and a ‘picture’ of these echoes was in the distribution of the matter. This will give some clues as to the formation at this time. The characteristic distance has been growing as the Universe expands, and stands at about 500 million light years (153 megaparsec) today.

Credit: Nature, September 28, 2016

Baryon Acoustic Oscillations

In a similar way that supernovae work as standard candles, baryon acoustic oscillations (BAOs) act like standard rulers. If you were to mark the position of enough galaxies and you can measure the apparent size of these BAOs. You then compare those measurements with the size predicted by their redshift, and you can work out how far away these particular BAOs are. By measuring the redshift of these galaxies, and plotting that against distance, it is possible to reveal how the expansion of space has behaved through cosmic history.

The best view yet of BAOs was revealed in July by a Sloan Digital Sky Survey program called the Baryon Oscillation Spectroscopic Survey (BOSS). This is the largest such galaxy survey yet. “This technique is really coming into its own,” says Saul Perlmutter, a physicist at the University of California, Berkeley, who led one of the teams that discovered dark energy in 1998 and who received a share of the 2011 Nobel Prize in Physics along with Adam Riess and Brian Schmidt for the work.

As well as backing up the supernova results with independent evidence that expansion is accelerating, the BOSS data give some clues about how dark energy behaves. And the pattern of acceleration suggests that if dark energy is changing, it is not changing very fast.

For now, that's a conclusion that seems to favour a candidate for dark energy known as the cosmological constant. In the 1920s, Einstein toyed with adding a constant term to his equations of general relativity—equivalent to giving empty space its own energy. According to general relativity, this cosmological constant would indeed oppose the force of ordinary gravity. Einstein originally tuned the value of the constant to create a balanced, static model Universe. But in 1929, Edwin Hubble showed that distant galaxies are receding from us, and astronomers realized that the Universe is expanding. Einstein ditched the constant. Now, however, with evidence that Universal expansion is accelerating, the cosmological constant has come back into contention.

Dark Energy and the Quantum

One popular way to explain where dark energy comes from is to invoke quantum theory. Random, small-scale quantum effects create energy even in the vacuum of empty space – so the more space, the more energy. But when you calculate the amount of this quantum dark energy there should be, “you get it wrong”, says Elisa Chisari at Utrecht University in the Netherlands. And not just by a little – by a factor of 10120.

Maybe. Recent observations of the universe’s expansion have shown that, when you use standard cosmic models to extrapolate the early universe’s expansion rate to the present day, the predicted value falls far short of what we actually measure. One explanation could be that dark energy is more complicated than we thought. “You get into all kinds of exciting, glorious models,” says Heymans – ones where dark energy evolves with time, for example, or interacts with dark matter in mysterious ways.

Energetic Mystery

The question is why the vacuum of space should have energy at all. Quantum-field theory posits a profusion of virtual particles that briefly come into existence and then disappear—a seemingly outrageous idea, but one that has allowed quantum theorists to make extremely accurate predictions of how ordinary particles interact. These quantum fluctuations are supported by the Casimir effect plus virtual particles could be behind dark energy's repulsive force.

But it's hard to make the numbers stack up. The vacuum energy needed to produce the observed cosmic acceleration is about 1 joule per cubic kilometre of space; the simplest version of quantum-field theory adds up the energy of those virtual particles to give a value about 120 orders of magnitude higher than that. Such dense vacuum energy would rapidly rip the Universe to shreds, and plainly that has not happened.

Perhaps scientists are missing something. As-yet-undiscovered particles could cancel out the energy supplied by known particles. But, although it is simple to devise a theory that makes the value zero, it is hard to almost-but-not-exactly cancel out a huge number to leave the small required value of vacuum energy. “The cosmological constant is an odd beast,” says Perlmutter. “It makes the theory seem bizarrely asymmetric.”

So, although the cosmological constant remains the front-runner, theorists have been busy devising alternative forms of dark energy. Some have created new theories of gravity, similar to general relativity, but generating repulsion on very large scales. Others posit some kind of space-filling fluid, sometimes called quintessence, which acts a little like the cosmological constant, but slowly changes in density. Whatever the answer, dark energy is key to opening a window on “a completely unexplored region of fundamental physics,” says Mark Trodden, a theoretical cosmologist and director of the Penn Center for Particle Cosmology in Pennsylvania, Philadelphia. Finding the answer would not only change the view of nature, but also foretell the fate of the Universe.

Star Surveys

A slew of projects are preparing to gaze more deeply into the dark enigma, and work out whether dark energy really has always been the same across the Universe. The Dark Energy Survey (DES) has already begun, using the Victor M. Blanco telescope in Chile to scan a swathe of the southern sky, observing supernovae and cataloguing more than 200 million galaxies. Early in 2017, an even bigger survey—the Javalambre Physics of the Accelerating Universe Astrophysical Survey (J-PAS) near Teruel, Spain—should start drawing its own 3D map of the Universe to reveal BAOs. It will cover much of the northern sky and analyse up to 500 million galaxies with an innovative instrument that uses 56 colour filters to reveal redshift.

Meanwhile, in western Canada, a very different instrument is beginning to take shape. The Canadian Hydrogen Intensity Mapping Experiment (CHIME) near Penticton in British Columbia is an unusual radio telescope built from a series of half pipes, like a giant skateboard park. It gathers radio waves from along a north–south line that sweeps around as Earth rotates to build up a picture of the sky.

CHIME is built to pick up the waves emitted by cool hydrogen gas. Like galaxies, this carries the imprint of ancient acoustic oscillations. It might even reveal BAOs better than galaxy surveys, because galaxies are the result of relatively complex processes, whereas gas follows the original sound waves more directly. “It is a very clean measurement,” says principal investigator Mark Halpern. And its simplicity makes the instrument relatively cheap at Can$10 million (US$7.8 million). “CHIME is a stunning bargain,” says Halpern, an experimental cosmologist at the University of British Columbia in Vancouver, Canada.

Over the next decade, spacecraft and giant ground-based telescopes with much bigger budgets are expected to join the hunt for dark energy. Between them, these big projects, along with DES and J-PAS, will wield four cosmic tools. As well as spotting supernovae and plotting BAOs, they will also measure gravitational lensing and catalogue clusters of galaxies. Clusters are pulled together by gravity, so their growth could reveal whether the force of gravity begins to change at large scales. And gravitational lensing, the bending of distant images by intervening matter, produces subtle patterns in the orientation of galaxies. Seeing how these patterns vary over cosmic time could reveal changes in dark energy.

Starting in about 2023, the Large Synoptic Survey Telescope (LSST) in Chile promises to find huge numbers of supernovae and pinpoint billions of galaxies to trace BAOs, cluster growth and lensing. “LSST will be like DES on steroids,” says DES director Josh Frieman. The European Space Agency's Euclid mission, scheduled to launch in 2020, will also make use of gravitational lensing. Above the blurring caused by Earth's atmosphere, Euclid's sharp vision will be better able to spot the orientation of lensed galaxies. It will also be able to collect near-infrared light that is blocked by the atmosphere. NASA plans to launch a similar mission, WFIRST, sometime in the mid-2020s. WFIRST will have even sharper vision than Euclid, because it is based around a larger, 2.4-metre mirror—a piece of optics donated by the US National Reconnaissance Office, which runs the country's intelligence satellites. “That was an unusual technology-repurposing game,” says Perlmutter.

A Burst of Ideas

Even with so many eyes on the sky, dark energy may remain elusive. So some astronomers are looking at more outlandish probes of the cosmos. Gamma-ray bursts (GRBs) are flashes of high-energy radiation from the distant Universe. Many are thought to be caused when the core of a massive star collapses to form a black hole or neutron star. At Stanford University in California, Maria Dainotti wants to use GRBs as a new type of standard candle. This seems like a difficult task because these bursts are infamously diverse, flashing and fading seemingly without any pattern. “If you have seen one GRB, you have seen one GRB,” says Dainotti. But in 2008, she found that among certain GRBs, for which emission falls to a plateau and then drops off again, a shorter plateau means a brighter burst (M. G. Dainotti et al. Mon. Not. R. Astron. Soc. 391,L79–L83; 2008).

Dainotti is cautious about using GRBs for precision cosmology just yet, partly because there is not yet a clear physical reason for her correlations. Researchers don't yet know what is going on inside GRBs when the star's core collapses—the high-energy emission could be generated by a rapidly spinning neutron star or by material falling into a newborn black hole.

But when the theory is better established, bursts such as this could illuminate the early days of dark energy. GRBs are much brighter than type Ia supernovae, so they could be used to see farther and trace expansion back to when the Universe was less than one billion years old. If dark energy is changing its nature, then that distant view may be crucial.

Gravity Drops In

It may be that a new kind of astronomy will be needed to crack the dark enigma. In 2016, the Laser Interferometer Gravitational-Wave Observatory (LIGO) collaboration finally announced the detection of travelling distortions of space-time known as gravitational waves, predicted by Einstein a century ago. A distinctive chirp lasting a fraction of a second was the echo of colliding black holes, which more than one billion years ago shook the fabric of space-time as the black holes spiralled in and merged with each other.

“With gravitational waves like these, we could measure distance,” says Stephen Fairhurst, a physicist at the University of Cardiff, UK, and a member of the LIGO collaboration. The shape of the waves reveals the mass of the black holes and the total energy emitted. Combine that with the strength of the waves as they reach Earth, and you can work out the distance.

However, plotting the expansion history would also require finding the redshift, which is trickier. It may be possible to find the host galaxy of one of these events and use its light to reveal the redshift, although the host could be one of many galaxies in a broad target area, because gravitational-wave detectors cannot yet pinpoint direction precisely. If all these many and varied tools find no change in the behaviour of dark energy, researchers may have little choice but to give up and embrace the cosmological constant. “The moment we find ourselves accepting the cosmological constant will be when theory makes a convincing predictive step,” says Perlmutter. For example, a theory might predict a new class of particles to curb the cosmological constant, and these particles might then be detected by the Large Hadron Collider at CERN, Europe's particle-physics laboratory.

Physicists are fond of weirdness, so most will probably be hoping for the other outcome: that most of the substance of our Universe is an evolving thing that is even stranger than vacuum energy. If the source of acceleration is found to be either a new energy field or a modification of gravity, the consequences will be profound. “It could cause us to rethink how gravity and particle physics interact,” says Trodden. Finding a particle-based description of gravity has obsessed theoretical physicists since Einstein. To finally manage it, we may have to abandon his cosmological constant for the second time.

This article was originally published as part of Nature Outlook: The dark universe, a supplement to Nature. The Outlook was sponsored by Eisai Inc. All Nature Outlook content is editorially independent of sponsors, unless explicitly labelled as promotional content.

Dark Matter

Dark matter could be normal baryonic matter if it were all tied up in stuff like brown dwarfs or in small, dense chunks of heavy elements. Most of the matter in a galaxy are stars so the unknown 'stuff' could simply be rocks and dead stars and what not, nothing exotic. These possibilities are known as massive compact halo objects, or "MACHOs". The most common view is that dark matter is not baryonic at all, but that it is made up of other, more exotic particles like axions or WIMPS (Weakly Interacting Massive Particles).

A larger percentage, about 80 percent, of the mass of the universe consists of some form of matter that we cannot directly observe in the electromagnetic spectrum. Because we know of its existence because of its gravitational effects but we can’t see it we call it dark matter. As I said it does not emit light or energy that we can detect. So how did we come to the conclusion of roughly 80 percent?

While studying the rotation of the stars in galaxies an astronomer Vera Rubin, an astronomer from the U.S., found that the rotation rate of the stars was far to quick for the estimated mass of all of the stars in that galaxy, for its gravitation to hold them together. One may wonder how you could estimate the mass in a galaxy but it can be done by estimating the stars based on the luminosity and distance of the galaxy. Planets and dust, which would give off no or little light, would only make up a very small percentage of the mass based on looking around our solar system and local stars. One could assume that all galaxies would be similar to ours and based on that then one would come to the conclusion that there is way too little mass in the galaxies for their gravity to bond the stars based on the rotational rate of that galaxies, so dark matter fills the blank.

Also evidence of this elusive substance shows up in gravitational lensing where gravity bends the light passing near the galaxy. This has been also supported by observing galaxies that have collided. The dark matter has been moved away from the center of the galaxies and combined. It is observed again through gravitational lensing.

The familiar and mostly visible material of the universe is known as baryonic matter, such as stars and planets, is composed of protons, neutrons and electrons. Dark matter may be made of baryonic or non-baryonic matter. To hold the elements of these galaxies together, dark matter must make up approximately 80 percent of its matter.

Some thoughts I have had about dark matter is perhaps stars could have formed around lumps of dark matter. Perhaps the core of black holes were formed from dark matter after all why wouldn’t dark matter clump like normal baryonic matter and be distributed in a similar ways. It has gravity and gravity is what structured our universe after the inflation distributed it.

Most scientists seem to think that dark matter is composed of non-baryonic matter. The lead candidate, weakly interacting massive particles (WIMPs), are hypothetical particles that are thought which have not been observed. According to the models they would have ten to a hundred times the mass of a proton, but their weak interactions with "normal" matter make them difficult to detect, hence the name. Neutralinos, massive hypothetical particles heavier and slower than neutrinos, are the particles that are supported by the standard model, though they have yet to be observed. The smaller neutral axion and the uncharged photinos are also potential placeholders for dark matter.

The Origins of Mass

What exactly is mass?

Well, we measure it often as weight if you happen to have a gravity well available to pull on it. We heard the Higgs boson is responsible for transmitting mass, but what produces it? Apparently, most of the mass is in the nucleus of the atom, electrons are quite small relative to a nucleus but, guess what, they can’t find most of it. When you add it up, it just isn’t there.

We all know that protons and neutrons are made up of particles called quarks and are stuck together with gluons. Gluons are massless, and it seems that the sum of the masses of the quarks inside protons and neutrons makes up only a small percent of the nucleons’ total mass. According to their results, the up quark weighs approximately 2 mega electron volts (MeV), which is a unit of energy, the down quark weighs approximately 4.8 MeV, and the strange quark weighs in at about 92 MeV. The mass of a proton is 1.6726219 × 10^-27 kilograms or 938.28 MeV. A proton is comprised of two up quarks and one down, for a total of about 8.8 MeV, so where does the rest come from?

Scientists are proposing that spin, mass, and other nucleon properties which result from the complex interactions of the quarks and gluons within are responsible for the extra mass, but how this works is unknown. A deeper understanding of the protons and neutrons is required it seems. So how can one look inside a nucleon? Use an electron Ion Collider (EIC).

Unlike an atom smasher, such as CERN, which hurtles nucleons, protons usually, at each other at near light speed, electrons have no complex structure within them and can be used to ‘see’ inside the protons more clearly. If I understand the process correctly it is like using the electron in much the same way as we use a photon in a microscope or perhaps even an interferometer. That last one needs to be verified.

An EID would accelerate electrons in one direction and protons in the other and smash them into each other. This would allow scientists studying quantum chromodynamics to look deeply inside the nucleus of an atom and to be able to study the strong nuclear force in more detail than with the Large Hadron Collider (LHC). With a combination of linear accelerators and synchrotrons they would achieve energy levels of 100GeV. This is much lower than the LHC but with the electrons colliding with the proton a different set of data is achievable. They expect to have an EIC in operation by 2030, the Thomas Jefferson National Accelerator inert Virginia, USA

How does it work??

The proton, as we know, is composed of quarks and gluons. Gluons are the carriers of the force that binds quarks together. Free quarks are never found in isolation—that is, they are confined within the composite particles in which they reside. The origin of quark confinement is one of the most important questions in modern particle and nuclear physics because confinement is at the core of what makes the proton a stable particle. The internal quark structure of the proton is revealed by Compton scattering, a process in which electrons are scattered off of quarks inside the protons, which in turn emit high-energy photons, which are detected in coincidence with the scattered electrons and recoiled protons. Using this technique measurements of the pressure distribution experienced by the quarks in the proton is available. There is a strong repulsive pressure near the center of the proton (up to 0.6 femtometres) and a binding pressure at greater distances. The average peak pressure near the centre is about 10³⁵ pascals, which exceeds the pressure estimated for the most densely packed known objects in the Universe, neutron stars. This work opens a new area of research on the fundamental gravitational properties of protons, neutrons and nuclei, which can provide access to their physical radii, the internal shear forces acting on the quarks and their pressure distributions.

Inside every proton is a high-pressure environment that is greater than the forces that are experienced at the heart of a neutron star according to some of the first measurements. The physicists found that the quarks are subjected to a pressure of 100 decillion Pascal (10³⁵) near the center of a proton, which is about 10 times greater than the pressure in the heart of a neutron star. Could the energy stored in this pressure represent the missing mass, I ask.

Compton scattering is where an electron hits an atom and not only ejects an electron but also a photon.