Cosmology

Is it possible that the expansion of the universe stretches space time ? Could this stretching lengthen light waves arriving from distant galaxies, thus contributing to a galaxy redshift ?

The expansion of the universe is exactly the stretching of space time which has the effect of causing distant galaxies to recede from us at a high velocity. Thus the redshift of these distant galaxies is due to the expansion of space. Note that the motions of nearby galaxies is dominated by gravitational forces and hence their redshift (or blueshift) is not due to the expansion of space. Top

We know that the Universe is expanding. This means that there is an enormous space vacant or freely available in outer space to have expansion. How is this free space generated and how is the matter created first to fill this space ? (Abeer Mishra)

This is actually a tricky concept. When we say the Universe is expanding, it is not that new empty space is being created; instead space, itself, is expanding. What this means is that the distance between two particles (that have no forces acting on them) slowly increases without the particles themselves moving. Top

Does the big bang theory or the steady state theory explain the origin of life ?

Most astronomers believe in the Big Bang, primarily because it provides a natural explanation for the 3K microwave background. Top

If the universe was allowed to evolve again from the beginning, then would it look the same as it is now ? (Dimitra Atri)

The universe can reach its initial state, if its presently expanding phase can reverse itself and it re-collapses under its self gravity. This can happen if its average density exceeds a certain critical density (corresponding to a closed universe). Present observations suggest that the universe is dominated by a repulsive cosmological constant term, which will ensure that it expands for ever ! However even if the universe were to recollapse, it would be extremely improbable for it to again evolve into something like the present universe ! One reason is that many massive black holes form and if it recollapses they would all merge and increase the entropy many many orders above that during its expansion phase. So,thermodynamics would prevent the re-emergence of a universe even remotely resembling ours !

The current thinking is that universes may keep forming all the time but their evolution depends strictly on their initial conditions (parameters !). Thus if our universe had a somewhat higher density it would have collapsed long ago. Again it is possible to envisage universes forming with different values for the coupling constants, different masses for particles and so on. Their evolution would be very different ! Most of them may not form stars, or planets or admit life ! Each such system is likely to be unique. So it is extremely unlikely that the initial conditions are such that we again have a universe very much like ours ! Top

Since galaxies have formed from the matter expelled during big bang, do they also die like stars ? (Sumit)

"Matter Expelled" is a wrong usage! What has happened is that the early universe was radiation dominated for about half a million years. The temperature meanwhile kept dropping as it expanded. When the background radiation had cooled to a temeparture of a few thousand degrees, electrons and protons recombined to form neutral hydrogen and the universe began to be matter dominated. Fluctuations in the matter density grew with the expansion and collapsed to form structures under the action of their self gravity. Gas clouds within these structures contracted to form stars when their central temperatures became hot enough to ignite thermonuclear reactions which kept them shining for millions or billions of years. Most of the star formation was in the early phase of galaxy evolution. It is believed that by and large the present rate of star formation is far smaller, on an average a solar mass star every year per galaxy. Of course, certain processes like tidal interaction between galaxies can trigger cloud collapse and star formation. As the stars which have formed in a galaxy will have to die out at sometime (even the faintest and lowest mass stars have maximal life time of a trillion years), when most of the stars die out, a galaxy will also die out eventually on sufficiently long time scales. In about ten trillion years, galaxies would mostly contain cold dense remnants of stars like white dwarfs, neutron stars or black holes. On still longer time scales like ~ 1019 or 1020 years, other processes would result in most of the objects of the galaxy, escaping from it while the remainder would collapse to the centre. Top

What are the basic differences between the physical nature of matter and antimatter ? (Dimitra Atri)

Matter is made up of atoms and molecules which are in turn composed of basic entities (fundamental particles) like electrons, protons and neutrons. Antimatter would be made up of antiatoms which would be constituted from positrons (antielectrons), antiprotons and antineutrons. An atom of antihydrogen would consist of a positron orbiting an antiproton nucleus. The only difference would be the positron is of positive electric charge and the antiproton of negetive charge (unlike hydrogen where it is the electron which is negetive). However, antiatoms would be electrically neutral like atoms and cannot be spectroscopically distinguished (Antiatoms and molecules would emit the same spectral lines). Individual antiparticles as such have opposite quantum numbers to that of the particles. When particles and antiparticles interact, they will end up in annihilation producing just high energy photons etc. A star made up of antimatter (an anti-sun) would also generate energy using nuclear reactions, but however antineutrons rather than neutrinos would be produced in such reactions. However the electromagnetic energy (light, heat, radiowaves etc.) would still be in the form of photons.(A photon is its own antiparticle). The chemical and other properties of antimatter would differ very little from that of matter. Top

Is it possible that there may be some distant galaxies which are made of antimatter ? (Dimitra Atri)

From the answer above, it is certainly possible to have stars, galaxies or other celestial obejcts made up of antimatter. However, we can be sure that such objects are very uncommon, if at all they exist. For one thing if they were as common as matter dominated objects, we would expect them to annihilate matter and produce vast amounts of high energy gamma ray radiation and other particles as a result. From the observed gamma ray background, we can put a limit on the existence of such antimatter objects.Again cosmic rays show only a very small fraction of antiparticles. Indeed it is one of the puzzles of cosmology why there is at least 109 times more matter than antimatter ! It is now believed that baryon non conserving reactions in the early universe gave rise to this drastic asymmetry ! Top

What is black light ? (Dimitra Atri)

There is no such thing as black light. There is dark matter and dark energy which could dominate the dynamics of the universe. They do not emit any visible radiation but manifest themselves only in their gravitational influence. Top

what was the aim of the COBE mission ? What were its results ?

The COBE mission (that is Cosmic Background Explorer), was a satellite launched to detect anisotropies (fluctuations in temperature) in the cosmic microwave background (2.73 degree K) radiation, a remnant of the hot dense phase of the early universe. This radiation was known to be isotropic (that is same radiation temperatures in all directions) to one part in 10 to the power 4. However as the universe is not homogenous but is 'clumpy' on various scales (that is local enhancements in density like galaxies,clusters of galaxies, superclusters and so on), the theory of the formation of the various structures from the growth of initial inhomogenities predicts anisotropies to a few parts in 10 to the power 5 on degree angular scales (or less). These temperature fluctuations dT as a function of polar co-ordinates (x,y) were mapped by COBE. Future missions such as MAP or Planck, would map these variations over much smaller angular scales putting constraints on various cosmologicals models. Top

What is gravitational lensing ? How does gravitational lensing help to remove or smear out the anisotropies in the cosmic background radiation ? (Sumit G)

As is well known, gravitational fields can bend or deflect light from a straight line path; the sun for instance can deflect light grazing its surface by angles of about 2 arc seconds. Over a long enough baseline this deflection can focus light rays, so that the sun can act as a gravitational telescope. Over baselines extending over cosmic distances, the distribution of matter in galaxies and clusters of galaxies can produce distortions and focus light coming from distant objects beyond them to form images and intensify the light. Over great distances these distortions can be large.

In 1979, gravitational lensing leapt from a theoretical curiosity to an important observational tool, with the discovery of 0957+561, a quasar split into two images, 6 arc seconds apart by the gravitation of an intervening galaxy. Gravitation lensing is an important tool in probing cosmic mass distributions directly, irrespective of whether it is composed of dark matter in the form of compact massive objects (MACHOS) in our galactic halo when stars in the large Megallanic cloud are in the same line of sight as the earth and the lensing object (forming a so called Einstein ring), giving rise to a characteristic intensity variability of the lensed object.

Gravitational lensing does not produce any significant effect in the cosmic background radiation. Top

Last updated on: February 20, 2024