Dark Matter and Dark Energy
In 1930s, astronomers noticed that in some galaxy clusters, some of the galaxies were moving extra fast than possible with available matter (and its gravitational force). The question was – Is there any matter which is not visible to us but exerts its gravitation responsible for keeping the galaxies put together? Again in 1970s researchers proposed that stars of Andromeda galaxy were moving so fast that there needs to be present some tremendous amount of matter which does not emit any electromagnetic radiation but exerts gravity. Since it does not emit any EM radiation, it’s not visible to telescopes and thus is called dark matter. Later, the scientists confirmed that dark matter does exist and is an important constituents of the galaxies and clusters of galaxies and puts them together. Further, it is now estimated that 80% of the matter in universe is dark matter.
The direct observational evidence of the dark matter comes from careful observations of the rotation rate of the galaxies. To scientists, the galaxies appear to be surrounded by a giant or galactic halo containing matter capable of exerting gravitational influence but not emitting any observable radiation. Further, it was also indicated that majority of a galaxy’s mass lays in this very large halo, which is around 10 times the diameter of the visible galaxy. For example, our own Milky Way galaxy contains about 100 billion stars and it is thought to have been surrounded by a dark matter halo that probably extends out to about 750,000 light-years. The mass of this dark matter halo appears to be about 10 times greater than the estimated mass of all the visible stars in our galaxy.
Albert Einstein had introduced a mathematical term into his equations to keep a balance between cosmic expansion and gravitational attraction. This term became known as the “cosmological constant,” and seemed to represent an unseen energy that emanated from space itself.
After Edwin Hubble and other astronomers showed that the universe was indeed expanding, the cosmological constant no longer appeared to be necessary, and so it was not seriously considered again for decades.
Then, starting in the 1990s, a series of discoveries suggested that the “dark energy” represented by the cosmological constant does indeed exist.
Current measurements indicate that the density of this dark energy throughout the universe is much greater than the density of matter—both luminous matter and dark matter combined.
Though astronomers have measured the presence of this dark energy, scientists still have no idea what causes this energy, nor they have a clue what this energy is made of.
The quest to understand the cosmological constant in general, and dark energy in particular, is one of the great unsolved questions in astronomy.
Composition of Dark Matter
But nobody has a real idea of what dark matter is and what these galactic halos are made of. However, there are some educated guess works divided into two schools of thoughts. One schools supports MACHOS or Massive Compact Halo Objects and another school advocates WIMPs or Weekly Interacting Massive Particles. However, no dark matter particle has ever been detected.
Implication of dark matter on shape of universe
- Dark matter in the universe exerts a gravitational pull in the expanding universe. The more dark matter there is in the universe, the more likely it would be that the universe would have a closed geometry, and that the universe would end in a Big Crunch.
- Continued expansion of the universe means that the total amount of dark energy keeps increasing.
- Since the total amount of mass in the universe is not increasing, that means that the expansive effect of dark energy will ultimately overcome the contractive effect of dark matter.
- The more dark energy there is, the more open the geometry of the universe will tend to be, and the faster the expansion rate of the universe will increase over time.