sábado, 26 de novembro de 2011

23. Future Experiments - Sean Carroll - Dark Matter, Dark Energy: The Dark Side of the Universe



In our effort to characterize the dark side, we've recently focused on where the universe came from, the laws of string theory, the Multiverse, extra dimensions, etc. These may help us to determine the natural values of dark matter and dark energy, but thinking can only go so far, so we need to go beyond our current understanding by means of experiments.

Special Relativity and Newton's gravity led us to General Relativity. Quantum mechanics and General Relativity led us to string theory. Yet experimentation will lead us to something even better. Our standard model of particle physics could be thought of as being too good. We need more experiments in order to be able to come up with something better. The concordance cosmology of 5% ordinary matter, 25% dark matter, and 70% dark energy is a victory, but we need much more to characterize the dark side.

When we learn more, there are more chances when sometime we may find great surprises. The supernovae observations surprised us with the accelerating universe. Who knows what other surprises await us? I like this point by Sean and I hoped he would expand on it much more than he did. Maybe in the last lecture of the course? This all points to the need for more observations.

So far, our observations have been strictly of photons, with only a minute fraction otherwise. Photons have enabled our discovery of energy density to explain the flatness, or dark matter to explain the structure, of the universe. But with improved resolution we can discover much more.

The CMB observations provided great returns when mapped out at a one degree resolution. But we can go to smaller scales for new returns. The Planck Surveyor in 2008 should tell us the amount of dark matter, ordinary matter and more sensitive fluctuations. The intrinsic polarization of the universe has been detected, but gravity waves from inflation may be detected. The somewhat vanilla predictions of flatness and homogeneity could be improved by tensor predictions from inflation.

Large scale observations are also improving. The SDSS returned one million redshifts and is complemented by 250,000 from 2DF. Combine these with the lensing surveys where weak lenses affect shapes of galaxies, and we have a map of all the stuff in the universe. The LSST around 2013 in Chile will observe the entire sky each night. This massive archive will need help from Google to just browse the catalog. Large scale structure, lensing, supernovae, and potentially dangerous near earth asteroids are all on the observing schedule.

Clusters of galaxies will help with the number density of the universe. The Sunyaev-Zeldovich effect is the shadow created when the CMB goes into a cluster, scatters off the gas, and does not then come to us. The spot left in the CMB is observable and is a good measurement of all galaxies in that region of space. The South Pole Telescope is one such project, but will need to be followed up by x-ray observations. Since dark energy actually prevents more clustering in the universe, the number of galaxies is a good way to measure the dark energy.

The last photon observations in our list, is of gamma rays. When dark matter annihilates there would be gamma rays produced. GLAST could detect them in late 2007.

Neutrinos are the first non-photon observations we'll talk about. So far only the sun and SN1997a have been observed in neutrinos. The nature of neutrinos is one of light mass and mixing into one another, thus solving the solar neutrino problem. But there could be new kinds of neutrinos like sterile that only interact with gravity and each other.

Gravity waves are weak but could be detected by LIGO, now that data is coming in at the intended rate. The LISA satellite observes a different wavelength than LIGO, where spiraling black holes could be used as the next standard siren. This is a clean idea without the messy problems of photon physics.

Dark matter could be detected on Earth by CDMS in a Minnesota mine or DAMA in Italy. The latter claimed a positive detection, but not many believe it. It could be a new kind of dark matter, which is the very point of making such observations to start with.

The Cassini mission has confirm the expected time delay of radiation caused by gravity. The STEP satellite is a high risk but high payoff experiment for testing equivalence. The inverse square law is being tested at sub millimeter scales.

Dark matter and dark energy need explaining from particle physics, which means going beyond the standard model. The axion or supersymmetry options for dark matter, and the quintessence, vacuum energy, or worse, options for dark energy, are all open to new particle physics research. Fermilab's Tevatron and CERN's LHC will open a new realm in 2008 for the Higgs Field, supersymmetry, or even extra dimensions and completely new phenomena.

We've set ourselves the task of understanding the dark side of the universe, the dark sector that makes up 95% of the stuff we seem to have in our contemporary cosmology. For the last few lectures, our strategy for understanding the dark sector, has been to think really hard about where the universe came from, and how inflation could have taken us from a tiny patch to the big universe we see, and to think about how the fundamental laws of physics work, in terms of the possible roll of string theory, extra dimensions, and the multiverse.

These ideas come into our conception of dark mater and dark energy, because they give us a framework in which to think about what are natural values for different things to have. What is the natural value for the dark matter to be, for the dark energy to be, etc?

Thinking hard is great, and sometimes it's all you have. Yet experiments and data are always better. In either case, what we're trying to do, is to go beyond our current understanding. Sometime you do that because your current understanding is mutually inconsistent with itself. Special Relativity and Newtonian gravity were inconsistent, then Einstein used that to invent General Relativity. Now we have quantum mechanics and General Relativity being inconsistent, and that's how we get to string theory.

Yet it's never as good as actually having a piece of data that says your current theory is wrong. Part of the reason why we have not had great progress in particle physics in the last 30 years, is because the standard model is too good! It fits all the data really well. To move beyond and get a better idea of what's going on next, it is always better to go a new, experimental piece of information, which doesn't fit our current theory.

So the good news is we have a huge amount of experiments coming online in the next few years, which hopefully will teach us more beyond what we already know. Sean reminds us, though we've probably heard it too often by now, that we have a model that fits the data. We have a picture of the universe in which 5% is ordinary matter, including particles from the standard model of particle physics, 25% is dark matter, and 70% is dark energy.

The dark matter is some particle that is massive, moves slowly, and doesn't interact very strongly, while the dark energy is some form of energy that is smoothly spread through space and slowly evolving through time. So that's fine, that's a model that fits the data we have. Yet should we therefor declare victory? If we stopped collecting more data, we wouldn't have any discrepancies!

Yet we want more than that. We want to understand what is going on. Even though we know there is dark matter and that there is dark energy, we don't know what these things are, so we're going to push our experiments forward. Remember that when we do so, sometimes we learn a little bit more about what we were studying, yet sometimes we're completely surprised.

One of the supernovae groups that tried to measure the deceleration of the universe, titled themselves as "measuring the deceleration." Yet they actually discovered the acceleration! Those are the best kinds of experiments. So even though we can talk about what kinds of experiments we'll be doing over the next few years, we cannot talk about what we'll learn from them. We have things we hope to learn, but we need to keep an open mind and maybe we'll be surprised.

So we can characterize the kinds of experiments we'll be doing, by the kinds of things we'll be looking at. In other words are they looking at photons, neutrinos, or what have you? Photons, of course, are a favorite way to look at the universe, they always have been. So we'll start by looking at what we can learn by measuring photons coming from the sky. There's also photons we can make here on the ground of course. Traditionally, all of astrophysics and astronomy, by default meant studying photons you get from the sky. Photons across the electromagnetic spectrum, from radiowaves at very long wavelengths, through microwaves and infrared, up to visible photons that we see with our eyeballs, and then to shorter and shorter wavelengths through ultraviolet, x-rays, and ultimately to gamma rays.

It says something about the state of our progress that we even make a meaningful distinction between looking at the sky in photons versus anything else! For the first time in human history, in the last couple of decades we've begun to look at the sky in other kinds of ways. Yet nevertheless, it's still photons that give us the overwhelming amount of our information.

You might think we could organize the kinds of experiments we're going to be doing, by what kinds of photons we'll be looking at, x-rays versus ultraviolet for example. However it turns out that for some kinds of objects in the sky, they are best understood by looking at the same object in different wavelengths. Therefor lets organize how we'll be looking at the universe by what we'll be looking at, rather than by what wavelength of photon we'll be using. So lets go from the largest things in the sky to the smallest.

The largest thing really, is the CMB, the leftover radiation from the Big Bang, a snapshot of what the universe looked like about 400,000 years after it began. It's the tiny variations in temperature that the CMB has in place to place within the sky, that teach us a tremendous amount about what the early universe was really like. It's already been a treasure trove of information about what the universe was like then, and what it must be like now. We've learned from the temperature fluctuations in the CMB, first that space is flat. Therefor, if the Friedmann equation is right, you need enough ρ to account for a spatially flat universe. Secondly, we've learned that ordinary matter is not enough. In order to explain the different sized splotches of hot and cold that we see in the CMB, it's necessary to introduce stuff other than ordinary matter and that of course, is the dark matter.

So what will we learn by going beyond what we've already done with the CMB? Well the first thing we can do is the most obvious, which is we can get better resolution, we can look at smaller angular scales. The most interesting angular scale, if you have to pick one on the CMB, is about one degree. That one, we've pretty much mapped out. Yet there could be surprises awaiting us at smaller and smaller scales. One prediction of inflationary cosmology is not just that there are perturbations at early times, but relationships between the amount of perturbations at different wavelengths, at different distances on the sky. So a big project for the future would be comparing fluctuations of large angles on the sky, to what fluctuations looked like at smaller angles.

To do that, we have a satellite supposed to be launched by 2008 called the Planck Surveyor, after the German astrophysicist Max Planck. It will give unprecedentedly accurate views of what the CMB looks like at very tiny angles. We'll get from that, a high precision way of measuring the cosmological parameters. Not just how much dark energy there is, but how much dark matter, ordinary matter, and the kind of fluctuations of the early universe which grew into the galaxies and structures we observe today.

Yet even then, we won't be done with the CMB. We're still going to try and squeeze more information out of it. We've already mentioned that we've discovered the polarization of the CMB. The wavelengths we're getting from the CMB, are polarized in different directions, depending on the temperature fluctuations. Yet in addition to the intrinsic polarization we've already observed, there's an additional component, hypothetically, due to gravitational weaves created by inflation.

These have not yet been observed. One of the major upcoming projects, hypothetically in a satellite mission, will be a dedicated search for the polarization imprint of gravitational waves from inflation. If you could find those, and they had the properties that were predicted by our currently favored models of inflation, that would be the best evidence we know how to get that something like inflation that was true in the early universe.

Right now we have models of inflation that are consistent with everything we observe, yet their prediction are kind of vanilla. They're that the universe should be flat, that the perturbations should be approximately the same on all scales. The tenser perturbations, which are what we call the perturbations you get from gravitational waves, are a specific and quantitative prediction from inflation. So if that turned out to be right, you'd really think we were on the right track.

After the CMB, the next largest thing we can look at is large scale structure in the universe. In other words we can map out the stuff that you see in the later universe, the stuff you see in clouds and dust. Right now the state of art is something called the SDSS (Sloan Digital Sky Survey), along with a slightly smaller survey called the 2DF (Two-degree Field Galaxy Redshift Survey). The SDSS has about 2 million redshifts for galaxies, collected at a telescope in Arizona, while the 2DF has about 250,000 redshifts collected from Australia.

Together they've given us unprecedented views of the way in which structure stretches across the universe. So by matching the CMB, which is a snapshot of the universe 400,000 years after the Big Bang, to such redshift surveys, which are snapshots of the universe 14 billion years after the Big Bang, we're beginning to piece together how structure has evolved from early times to late times.

That is a messy and complicated astrophysical problem. Even if you knew from inflation what the initial perturbations were, and you knew the ingredients of the universe in terms of dark matter and dark energy, it's still quite a bit of work to trace, from those initial perturbations, to what we see today. So there's a lot of dirty, important astrophysics that needs to be done, and observations of the distribution of structure are going to be crucial to make sure we're on the right track when doing that.

One of the most important roles of redshift surveys of galaxies across the sky, will not simply be to use those data all by themselves, but to compare them to other kinds of data. So one thing we'll be doing is surveys of gravitational lensing. Only very recently have we been able to develop cameras that can take images of what's on the sky, to such precision that you can tease out the slight perturbations of the shapes of galaxies due to gravitational lensing. It's the weak lensing that's going to be a treasure trove of information for future cosmologists. Remember that if gravitational lensing happens, the beam of light we get from a background galaxy is reflected by some mass distribution, and by the amount of deflection you can measure how much mass there is. That's more or less the basic picture if you're lucky enough to get the object acting as a lens, one background object that you observe on the sky.

The real universe of course, has many galaxies in it, many clusters of galaxies, all in a messy configuration. That's bad if you only want to take one picture and get one piece of information, but we're learning how to use that messiness to our advantage. We're now learning how to make 3D maps of where the stuff is in the universe, using the amount of gravitational lensing that is undergone by galaxies at different redshifts. So by combining redshift surveys that tell us where the galaxies are in space, with lensing surveys that show us how their images are distorted on the sky, we're mapping out where the stuff is in the universe.

This is something first done in 2007, a technique we'll be perfecting and improving just a few years from now. The great leap forward in doing these kinds of surveys will be something called the LSST (Large Synoptic Survey Telescope). This has been on the drawing board for quite a while now, and will be for some time to come. It is supposed to start construction in 2010, be finished by 2013, being built in the mountains in Chile where the air is very thin for the best view of the sky. The goal is to survey a large patch of the sky every single night.

So what something like the SDSS can do over the course of years, is to take images of different patches of the sky, and piece them together into an image of what is going on. The LSST will basically be doing that everyday. So it will be collecting a huge amount of data, unprecedented in astrophysics, over a petabyte of data per year! So just as a gigabyte is 1000 megabytes, a terabyte is 1000 gigabytes, and a petabyte is 1000 terabytes. So one petabyte is the equivalent of 250 billions songs on your ipod, a huge amount of data, so much so that just surfing through the data, looking for different pieces of information, is an incredibly difficult computer science problem.

In fact, we'll not be surprised to learn that scientists working on the LSST project, are collaborating with researchers from Google, who are the best people in the world for searching through large databases and finding things. So astronomers want Google's technology to search through their database, while Google wants the data astronomers have, because they are the only ones in the world who collect huge amounts of data and then give it away for free.

So what we'll be getting from LSST is huge amounts of data, where lensing will be an important goal to be able to look for the 3D structures of the universe. Yet we'll be looking at the same region of the sky, over and over again. In other words we'll be collecting data in the time domain, something astrophysicists have traditionally not done because it's just to expensive. In other words, we'll not just get weak lensing and large-scale structure, but also discover supernovae and asteroids coming to crash into the earth. This is a major funding source for the LSST, to find objects in the solar system that might be of danger to us. It doesn't tell us anything about dark matter and dark energy, but it's still a useful goal.

So going to smaller and smaller things in the universe, next lets consider clusters of galaxies. Remember that clusters are, on the one hand, the largest bound structures in the universe, yet on the other hand they are a fair sample of what is in the universe. So by studying clusters so far, we've been able to learn about the relative proportions of lets say, dark matter to ordinary matter in the universe.

The new thing we'll start doing with clusters of galaxies, is to count them. By counting the number density of stuff in the universe, you're tracing out how it has evolved. People have always known this to be true, yet historically this is a very difficult technique to use, since when you're counting things it's crucially important that you don't miss anything. In astrophysics, some things are bright, some are dim. When you try to count things, you don't know you've found all of them, but clusters of galaxies are special.

In a cluster, galaxies fall together, gas and dust falls in along with them, and it heats up. There's something called the Sunyaev-Zeldovich effect, in which it's not that we're looking at the gas, but at the shadow cast by this hot gas, on the CMB. So you look at the CMB in the sky, and look for a little dot on it, a little dark spot. This is caused by the fact that the photons from the CMB come into the cluster, scatter off the gas, and don't get to you. The reason why this is important is that since you were looking at the light emitted by the cluster, it would become dimmer as you get further away. So it's harder to find clusters that are far away.

Yet if you're looking at the shadow cast by the cluster, it's just a shadow, so it doesn't get any dimmer the further away you get! So by doing a survey for the Sunyaev-Zeldovich effect, we'll be able to find all the clusters of galaxies in the patch of sky on which you're looking, which will be an unbiased sample of all the clusters of galaxies in some region of the universe. We are currently doing that with something called the South Pole Telescope, a ten meter radiotelescope set up in Antarctica in the south pole, that starts operation in 2007. Once it finds these clusters of galaxies, we'll follow that up with x-ray observations that will allow us to weigh the clusters, to measure how much mass is in them.

Yet more importantly, it's the number of clusters of galaxies that count. As the dark energy making the universe expand, takes over, it stops structures form forming. When you have a matter dominated universe, the matter gradually collects together and forms bound structures. Yet when the dark energy takes over, the matter is pushed apart and galaxies stop falling together. So the number of clusters of galaxies as a function of distance in the universe, is a very sharp way of figuring out how much dark energy there is. It is arguably competitive with supernovae and other measurements. We don't know yet, since we don understand the fundamental physics of galaxy clusters very well, but that's something that numerical simulations are teaching us about right now.

Meanwhile of course, we're not done with supernovae. They are the way we discovered the dark energy in the first place, and we want to do better. The important role there is that we'd like to understand the fundamental intrinsic nature of the supernovae themselves. If you collect enough examples, so that instead of 20-40 supernovae, you have 1000, then you can start talking about what the distance-redshift relationship is for each individual possible kind of supernovae. What it looks like for supernovae in elliptical galaxies, spiral galaxies, and so forth. You can search for possible systematic effects that might put you on the wrong track.

By doing this, we'll be able to measure the equation of state parameter of the dark energy. It's what we called w, and will be -1 if the dark energy is truly constant, and something like -0.8 or -0.9, if it's a gradually declining quintessence. So looking for supernovae at very high redshift, is our best hope for figuring out whether the dark energy is truly constant or slowly varying.

Finding supernovae at large distances is hard if you're here on earth. Therefor there are proposals to build a dedicated satellite to do exactly that. Right now the plans are for NASA and the DOE to team up and build JDEM (Joint Dark Energy Mission). However Sean can't promise us it will be built, since these things depend on Congress, money, and other things that scientists don't have much control over.

Finally in terms of looking at photons, we can look at gamma rays for indirect evidence of dark matter. That is to say, we have dark matter particles in the universe, so we can of course look for them here on earth, but also in the sky by waiting for dark matter particles and anti-particles to annihilate. If the dark matter is a WIMP, then the real dark matter density comes from both particles and anti-particles. Occasionally they are going to annihilate and give off gamma rays. Right now we don't have much of a capability for looking at gamma rays in the sky, but the new GLAST satellite is scheduled to be launched in late 2007, which will give us our first direct view, and might be an indirect detection of dark matter.

Of course those are not the only particles we use. We don't only use photons to look at the universe, we use other particles like neutrinos. For example, we use neutrinos. Ever since the solar neutrino problem that was noticed in the late 60s and early 70s, we've been able to find neutrinos from outer-space, not just from here on earth. Besides the sun, the one kind of neutrino we've been able to detect from outer-space are neutrinos from SN1987a.

So one of the things that neutrino physicists have been doing, is planning for the next supernova. They're ready to get even more information about fundamental neutrino properties from whatever supernova goes off next in our own galaxy. We don't know when that's going to be, but it typically happens once every hundred years, so you never know.

Meanwhile we'd like to learn more about the fundamental nature of neutrinos, which has recently opened up as a growth field in physics. We now know the neutrinos have mass, they are very light, but do have some mass. Because they do, they can mix. In other words, we said there were things called electron neutrinos, muon neutrinos, and tau neutrinos. Yet they all can change into one another as long as they have mass. This has been experimentally found, and is the kind of thing we're trying to better understand.

The role for dark matter is that none of the three neutrinos we know and love, can be the dark matter. Yet there could be others, kinds we haven't yet directly detected. For example, sterile neutrinos which do not interact with W and Z bosons. The only way sterile neutrinos interact is through gravity and with other neutrinos. So we're trying to build neutrino detectors that will be able to tell the difference between sterile neutrinos and ordinary neutrinos, since that might play an important role in the dark matter mystery.

Getting more exotic, we can look at gravitational waves. Just like electromagnetic waves arise from electric charges moving up and down, gravitational waves are created when heavy objects move back and forth. For example, if you have two objects orbiting each other, they will be emitting gravitational waves.

Now, gravity is a very weak force, so these waves are not very easy to notice. We have never yet directly detected gravitational radiation. We do have an observatory that is trying to do exactly that. We have LIGO (Laser Interferometer Gravitational Wave Observatory), which finally right now by 2007is finally collecting data at the rate it was designed to do. It has not yet detected any objects, and it may not even succeed in doing so.

We have a batter plan for a gravitational wave observatory in space, called LISA (Laser Interferometric Space Antenna). This would include three satellites, 5 million km apart from each other, that look for gravitational waves in a completely different wavelength than LIGO was looking, which is actually good news for cosmology. One of the things LISA will be looking for, is if you have a tiny black hole orbiting a supermassive, million solar-mass, black hole, it will be orbiting, spiraling, and giving off gravitational waves all along.

It turns out that just like supernovae are standard candles, in-spiraling black holes can be standard sirens! You can learn enough about the parameters of the black hole binary, to figure out how much gravitational waves were given off at the source. So by detecting them here and measuring how strong the waves appear to us, we can figure out how far away it is. This is a completely independent way of measuring distances in cosmology. It's also a very clean way, one that doesn't get messed up with the messy astrophysics that ordinary objects giving off photons do. If we can turn standard sirens into a useful cosmological tool, we'd be able to get unprecedented precision in mapping out the expansion of the universe, out to very distance redshifts.

Next up we have dark matter. Remember that we can look for this by using photons, at the gamma rays that are emitted when dark matter and "anti-dark matter" particles come together. Yet of course, we can also look for dark matter right here on earth. We can build underground detectors that will wait for a dark matter particle to come in, bounce off a nucleus in the detector, and leave a little bit of energy there. We have detectors working right now, though unfortunately we don't have a very precise prediction for when we'll be able to observe them. There's a lot of freedom in the predictions there, so we can't say by which year we should be able to detect things, but it could very well be any moment.

There's a detector called CDMS (Cryogenic Dark Matter Search) which is underground in Minnesota and working right now. It currently has the world's best limits. There's another detector called DAMA is Italy, which works on an interesting system where it doesn't look for the total amount of signal, but for the variation in signal throughout the year, as the earth revolves around the sun, and moves through the dark matter cloud around it. We mention DAMA because it has claimed to have found a signal. They claim they have already detected the dark matter. Yet the reason why this isn't earth shattering news is because most people don't believe them. The current consensus in the dark matter detection communities is that they have some systematic error they haven't yet detected.

Yet there is an interesting alternative possibility, in that there could be a new kind of dark matter, some specific particle physics model which shows up more easily in something like DAMA than something like CDMS and the other detectors. In other words, this is why we do experiments, since the real world might be more complicated than the simple models we write down! So stay tuned for news from dark matter detection experiments, we might be very quickly learning important feature about 25% of the universe.

So coming in closer to home, besides looking at things in the sky, we can also do experiments closer by. Before, ordinarily, closer by would mean here on earth in the lab. Yet these days you have to extend it to at least the solar system. We can send satellites throughout the solar system to help us do physics experiments. The most obvious example is testing General Relativity. We already mentioned the Cassini mission which had as its primary goal, to take pictures of Saturn, Jupiter, and the other outer planets, yet it also tested General Relativity by sending signals to us that we used to measured the time delay.

There are new upcoming missions that will also test General Relativity. There is something called the STEP mission (Satellite Test of the Equivalence Principle), which will be asking if different objects made of different stuff, move differently in a gravitational field. Remember a very firm prediction of General Relativity is that everything responds to gravity in exactly the same way. That is known as the equivalence principle.

If there is some fifth force, some new light field that stretches out over macroscopic distances, it will show up as a violation of the equivalence principle. Quintessence might be exactly such a field, we don't know. Yet if quintessence exists, it's a very light boson that could give rise to forces that violate the equivalence principle. So experiments like STEP will be looking for exactly this type of thing. It's an example of a high-risk, high-payoff thing. Probably they will not see anything, yet if they do, it would truly change the way we think about the forces of nature.

Of course we're doing similar things here on earth. We're looking for new forces, and for new features of old forces. So we're doing experiments here on earth to look for violations of the equivalence principle, looking for different objects being accelerated by different amounts, in the gravitational fields of the sun or earth.

We're also looking for violations of the inverse square law of gravitational dynamics. When we talked about string theory in extra dimensions, we mentioned the possibility that some of these extra dimensions could be pretty big. They could be as large as a mm across. Right now we've been able to squeeze down the limit on the size of the extra dimensions, down to 1/10th a mm. We're going to continue to try and squeeze that down more and more.

This is again an example of an experiment for which if you find something, you absolutely win the Nobel Prize! You've found something incredibly important in the history of physics. The smart money bets they won't finds anything, but you don't know until you look, so we need to have some experiments that spend their time on the high risk projects.

Finally we'll emphasize the role of particle physics experiments in the future of cosmology. We've been talking about dark matter and dark energy, 95% of the universe. The evidence we have for dark matter and dark energy, comes from looking at the sky, by doing astrophysics, taking observations of galaxies and clusters, the CMB, the supernovae, and the whole universe all at once.

Yet ultimately, the explanation for dark matter and dark energy, will have to come from particle physics. The dark matter is some kind of particle, whether an axion, a supersymmetric particle, or something. The dark energy is even more mysterious, either quintessence, vacuum energy, or something even worse. To get a good idea of which of those models is more plausible than the others, we need to understand particle physics beyond the standard model. So doing lab experiments here on earth to increase our understanding of particle physics, is probably the single most promising way to learn more about the dark sector.

Right now we have the Tevatron at Fermilab just outside of Chicago, as the highest energy particle accelerator on earth, yet later in 2007, the LHC (Large hadron Collider) as CERN outside Geneva, will turn on. Starting in 2008, CERN and the LHC will be doing high-energy particle physics in a new realm we've never seen before. We're looking in exactly the regime where we should be able to find the Higgs boson, supersymmetric particles, and other particles just beyond the standard model. Not only are we looking for things that might very well be dark matter, and therefor 25% of the universe, but we'll also be hoping to learn things about sypersymmetry, extra dimensions, and other phenomena that come into the discussion when we talk about dark energy.

Now Sean admits the latter to be a very vague construction, not promising anything specific right there. Yet the point is that dark energy is so mysterious, we need all the help we can get. There's no known way that we can think of, to do a particle physics experiment to detect dark energy or see exactly what it is. Yet by learning more about the big picture, something that seems very unnatural to us, might suddenly make sense. So we're extremely hopeful that particle physics over the next five years, will be surprising us, teaching us new things about the fundamental nature of space, time, and matter, which ultimately will illuminate the nature of 95% of the universe we live in.

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