Here is some information. Hope this helps u. The first part is about dark energy. Read the second part too, regarding dark matter. But remember both are separate topics and have no significant relationship.
DARK ENERGY
Well, we all know that universe is expanding. In 1929, Edwin Hubble observed that the galaxies are moving apart from each other. This suggests that the universe was born as a result of a great cosmic explosion called the Big Bang.
About 15 billion years ago the universe was created expanding from nothing to 2 billion billion Km in just a single second.
The state of nothingness is called singularity. At this point, the space- time fabric was wrapped infinite times. Accordingly, nothing can known about what happened before the big bang or what was the cause of big bang. Hence it is still not clear why the Big Bang occurred, but scientists have predicted what would have happened in the first few moments:
·After 10-43 seconds the temperature of the infant universe was 100 000 billion billion billion0C. The universe expands rapidly and is filled with radiation mainly light and heat. Gravity appears as a distinct force.
·After 10-32 seconds the expansion slowed down and quarks, the smallest known particles appeared and started to combine and form bigger sub atomic particle.
·After 10-5 seconds the subatomic particles combine to form protons and neutrons, the two components of the nuclei of atoms.
·After 102 seconds the temperature drops to 1 billion 0C. Space filled with protons, neutron and electrons. Over the next 32 000 years, protons and neutrons react with background radiation to combine and form nuclei of hydrogen and helium- two simplest chemical elements.
·After 1 billion years the universe becomes transparent and its temperature drops to 40000C, low enough for complete atoms to form. These become pulled together by gravity, creating clumps of matter.
·After 2 billion years the first stars and galaxies begin to form from clouds of gaseous hydrogen and helium.
Since then universe is constantly expanding due to its momentum.
Now it is expected that gradually this expansion should slow down due to gravity. But on the contrary it is accelerating further. Bewildering. Isn’t it?
This weird phenomena was observed first in 1998 when a lot of super novae were observed and on observing a particularly a far off one, Supernovae Ia, it was seen that they were dimmer than expected. This could only be possible if the universe was expanding at a faster rate.
The supernova evidence suggests that the acceleration kicked in about 5 billion years ago. At that time, galaxies were far enough apart that their gravity (which weakens with distance) was overwhelmed by the relatively gentle but constant repulsive force of dark energy. Since then, dark energy's continuing push has been causing the cosmic expansion to speed up, and it seems likely now that this expansion will continue indefinitely.
Now what was this weird form of energy that was doing this work cannot be explained as yet.
Where some scientists say that this is a totally new and weirdest form of energy ever heard about, some say that it is not totally unknown, only its nature, properties and magnitude are unknown. It is said that when Albert Einstein gave his General Theory of Relativity in 1915, he thought that the universe was static. But in his calculations he accidently came across some “anti gravity” effect that was acting against the force of gravity. In order to accommodate this concept with his “static universe”, he gave it the name of “Cosmological Constant”.
Later when Hubble discovered that the universe was ever expanding and that it was not static at all. Einstein, when he came to know about this expansion, said that it was his biggest blunder.
Now the case is what is dark energy actually? Is it the Einstein’s Cosmological Constant or the Quintessence or some form of energy percolating from the apparent empty universe?
Quintessence:
Now, related to the Cosmological Constant, some scientists believe that the dark energy is some undefined form of energy percolating from the space which is considered empty (vacuum). The laboratory experiments show that the space which is considered a vacuum is actually filled of particles that wink in and out of existence.
Now this theory has a problem too. The problem is that this seeping vacuum- energy is calculated to be so powerful that by now the whole of the universe should have been blown off by now. Now, since the effect is not so strong therefore there is only one possibility, that this energy is not constant as Einstein had predicted, but weakens over time.
This leads to a new idea called Quintessence or the fifth essence. This represents a repulsive force in the space unlike gravity or a magnetic field.
This energy is said to have been born with the gravity and the forms of forces, at the time that universe was born. Later the two most dominant forces- Gravity and Quintessence got engaged in a sort of fight of power, in which quintessence emerged victorious and started pushing the galaxies and other celestial bodies away from each other and universe started expanding.
This seeping vacuum energy is said to have a weird effect called negative pressure.
Negative pressure:
It said earlier that dark energy is an anti- gravity force, however it does not act exactly opposite to gravity. It does what the general theory of relativity tells it to do, if it has negative pressure.
If you think in terms of the universe as a very large balloon, when the balloon expands, that makes the local density of the [dark energy] smaller, and so the balloon expands some more …. because it exerts negative pressure. While it’s inside the balloon it’s trying to pull the balloon back together again, and the lower the density of it there is, the less it can pull back, and the more it expands. This is what happens in the expanding universe.
This negative pressure increases in importance and impact as the universe expands and gains more acceleration.
To understand what pressure has to do with gravity let us look at Einstein’s Theory of Relativity.
According to this theory matter is not the only source of gravity, there are two other sources:
·Energy, which is interchangeable with mass according to E=mc2
·Pressure
Stars more massive than the sun must exert an even stronger pressure to counterbalance their gravity. For a star greater than about four times the sun's mass, the counterbalancing pressure becomes as strong as the density of the star. When this happens, pressure contributes as much as mass does to the gravitational force, Einstein's theory says. In effect, the gravitational pull inward drastically increases.
The more the star contracts, the greater its pressure and density, and thus the stronger the gravity. Unable to resist, the star undergoes a runaway collapse, and its gravity becoming so strong that not even light can escape its grasp. A black hole is born.
The contribution of pressure is "an aspect of gravity that was there all along," notes Turner. He says that anyone who accepts the reality of black holes has implicitly accepted the notion that pressure can be a key source of gravity.
According to Einstein's theory, pressure has another mind-bending property: It can be negative. An object having negative pressure resists being stretched. Think of negative pressure as silly putty or a rubber sheet. The atoms don't want to be drawn apart.
It's counterintuitive to think that a material such as rubber, which draws itself inward when stretched, could push objects outward. Yet if dark energy's antigravity effect—its ability to exert negative pressure—were strong enough, it could swing the gravity meter from the plus side to the minus side, Einstein's theory dictates.
Gravity normally pulls matter together. Instead of pulling, dark energy would cause gravity to push. Instead of tugging and slowing the expansion of the universe, dark energy would rev it up.
Scientists are relying on the satellites designed to map the Cosmic Microwave Background (CBM) to find out about the expansion of universe and the dark energy.
Now what is this CBM? This, Cosmic Microwave Background in some form of radiation that is aftermath of the Big Bang. It is found that CMB is slightly hotter where there are more galaxies. The scientist found a fingerprint of dark energy in the CMB and that’s the only answer to their argument about the weird force that accelerates the expansion of the cosmos.
The new findings put it on firmer footing, being completely independent of the supernova observations. They show that dark energy influences the light particles, or photons, of the CMB, which radiate throughout space. When a photon flies past a concentration of mass, such as a galaxy, it falls into a gravitational well, like a ball rolling downhill, and gains energy. As it climbs out of the well, the photon loses precisely the same amount of energy. Or at least it would, if all it encountered were normal matter. But dark energy, being gravitationally repulsive, makes a gravitational well shallower as a photon passes through, so the photon exits with slightly more energy than it had when it entered. This makes the CMB hotter where there is more mass - where there are galaxies, in other words.
This, Scranton and colleagues reason, makes the CMB hotter where there is more mass - where there are galaxies, in other words. There is one crucial assumption in this argument for dark energy: it presupposes that space is flat, on average, rather than curved. There are good empirical reasons to believe that this is the case!!
Programs and schemes to get in the details of Dark Energy
One of the most initial programs, JDEM (Joint Dark Energy Mission). It is a joint program initiated by NASA and DOE. Earlier the JDEM was known as SNAP (Supernova acceleration Probe Mission). The mission is designed to explore the physics of dark energy, one of the great mysteries in cosmology that have come up in the last five years.
The expansion of the Universe implies the existence of an energy field which effectively gains energy as the Universe expands. We have no current physics idea about where this comes from. As always in science, questions lead to more questions. Other scientific aspects of JDEM include looking at gravitational lensing. There will be an optical telescope in space and what is basically a large camera that takes pictures of the sky. SLAC (Stanford Linear Accelerator Centre) will provide most of the electronics that make the camera work, as well as the flight software and flight computer that will control the whole unit. Currently there are only a few people working on JDEM, but it is predicted that 20 people will eventually work on this significant project. The estimated launch date is 2014.
The second program is the LSST (Large Synoptic Survey Telescope)- another way of learning about the intriguing field of dark energy and dark matter. LSST is a very large ground-based telescope that can take pictures of the sky over a very wide field, and is a joint collaboration of NSF (National Science Foundation, US) and DOE (Department of Energy, US). LSST will survey the entire sky every few days down to very faint levels. The science here is to use the distortion of background galaxies due to gravitational lensing. There is dark matter in the Universe and the light from distant galaxies propagates to us. It gets slightly bent by the intervening gravitational matter which you can see by looking for correlated distortions in the sky. Those distortions will tell you about the dark matter and the clumping of the dark matter. How concentrated it is tells us about the expansion history of the Universe.
DARK MATTER
The universe, apart from containing normal matter (dust, rocks, stars, galaxies, planets etc.) contains dark matter. These are the stars which were too small to ignite. All galaxies contain a certain amount of dark matter. These are referred to as Massive Compact Halo Objects (MACHOs). MACHOs have been detected through their gravitational effect on light, although there is no definitive knowledge of exactly what they are. The matter which we know is called baryonic matter but the dark matter can be considered a non- baryonic kind of matter. So far what we have found out in connection with dark matter is both baryonic and non baryonic. The baryonic dark matter can be considered as stars that do not give out sufficient light e.g. Brown dwarfs.
Now if the non baryonic does really exist, as we think it does, then it should have mass, occupy space and must have certain volume, as the definition of matter goes. It must also be numerous. If all this true then a large part of universe may contain this dark matter, as we think it does. Today, scientists estimate that that nearly 95% of our universe is dark matter; however they are still not able to identify it or its properties.
Black holes can be a common example of dark matter. Dark matter is not visible because either it does not emit light (electromagnetic radiations) or because it does not reflect light (electromagnetic radiation). This is what a black hole does. But the problem is that detection of a black hole is not an easy task. Till now scientists have only been able to detect massive black holes in large galaxies like Milky Way and others. Only one small elliptical galaxy M32 has a black hole.
A black hole can become a massive black hole it takes a very little time. To grow bigger and bigger, the seed black hole has to swallow a lot of gas and massive stars. In fact it takes only a fraction of total time- in which the universe was formed- for a black hole to turn a more massive one, may be billions of time of the mass of our sun. In front of such a massive black hole light has no chance of escaping.
Also there are the ghostly neutrinos. They are particles belonging to the family Leptons.
Leptons:
Leptons are a family of fundamental particles that make up the matter. Leptons have six kinds of particles:
·Electrons
·Muons
·Taus
·Electron neutrino (ne)
·Muon neutrino (nµ)
·Tau neutrino (nτ)
The other families are Quarks (Down, Strange, Bottom, Up, Charm and Top) and Bosons (Photons, Gluons and Weakons or weak bosons).
Quarks combine to form yet another family Hadrons. They consist of two types of particles- Mesons and Baryons (Protons and Neutrons). Baryons make up the normal or baryonic matter.
Neutrinos were discovered by Wolfgang Pauli, in 1930. Neutrinos are so small that they do not interact with the normal matter at all. They are practically not present for us. Earth is nothing for them, transparent. Hence they can make up a small part of the dark matter.
Antiparticle and Antimatter:
All particles have their opposites. They are exactly alike the particle but they have opposite charge e.g. Positron is anti particle of electron, anti quark is anti particle of quark. Anti matter is the opposite of the particles that make up the matter. It consists of non baryonic particles, which can be considered as dark matter.
The universe, therefore, is filled with normal matter (which we know), dark matter (no clue yet, only assumptions) and dark energy (the most weird form of energy ever experienced), however it appears to have mass due to E=mc2. All these arguments also say that our universe may be a flat thing.
Well, this all about the report and let’s see what more the scientists say about dark energy and dark matter.
2006-07-12 16:51:29
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answer #1
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answered by know it all 3
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