Usne Javaani mein ek baar Once in his youth |
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In the great tradition of science we begin our journey by sifting through various pieces of puzzle presented to us by Nature. Observations provide the input which have to be collated and interpreted by theory. The interplay of observations and theory is what makes science the ultimate source of knowledge.
Galaxies are conglomerations of tens of billions of stars and are separated by millions of light years from one another. When we look out at other galaxies we find that their light is shifted towards lower frequencies. This effect is known as redshift. To measure such frequency shifts we need some standard frequencies with reference to which the shifts are observed. Such standard frequencies are provided by the atomic transitions, which we believe obey the same laws of physics everywhere in the Universe.
Edwin Hubble (1920) discovered the law which relates the redshift for a given galaxy to its distance from us. This law is easily stated as
For a real universe such a simplistic picture does not hold. The recession
velocities are not constant in time but evolve with the universe. The approach
to a past singularity is strictly an extrapolation of the expansion law
observed today. But as we will see later, our Universe is believed to have
originated from a very hot and dense state. This paradigm is also knows
as the Hot Big-Bang model of the Universe.
The correct theory of gravity was discovered by Albert Einstein in 1915 and is known as the General Theory of Relativity (GTR). The most important idea in this theory is
Due to the non-Euclidean geometry around massive bodies straight lines can behave in unusual manner. For example, light rays, which also follow the geodesics, follow a curvilinear path when traversing through space close to massive bodies. Such rays, even if they started out parallel to each other far away, can intersect with one another. The bending of light rays was the first test of the General Theory of Relativity and demonstrated that it was the correct theory of gravity.
Just like massive particles lose their kinetic energy while climbing
out of a gravitational field, light photons lose their energy which manifests
as a shift in their frequency towards red. This effect is known as the
Gravitational Redshift .
The Universe appears to be very homogeneous and isotropic on large scales. This is a very fortunate situation since it simplifies the geometry of the universe considerably. With these conditions the Einstein's General Theory of Relativity predicts that the universe is described by a geometry with one free parameter describing the curvature of the universe and one unknown function: the scale factor of the universe. The curvature term is related to the total amount of matter in the universe depending on which we can have a flat, hyperbolic or elliptical spatial geometry.
There is a critical amount of matter density which decides the ultimate fate of the universe. If the total matter density is greater than the critical value the universe will ultimately stop expanding and start contracting. The corresponding spatial geometry is elliptical. For density less than the critical value the universe keeps on expanding forever and the spatial geometry is hyperbolic. For the critical case which separates these two geometries the universe expands forever and the geometry of space is flat.
The scale factor fixes the physical scales between two coordinate points. For most purposes the matter in the universe can be described as an ideal fluid. The details of how the scale factor varies with time is decided by the equation of state for this matter fluid. In particular it depends upon how the pressure of the fluid is related to the energy density of the fluid. Since the redshift of cosmological objects is related to the cosmic time, the distances to these objects are decided by not only the overall geometry but also on how the scale factor varies with time. Therefore the distance information up to a given redshift gives us clues about the state of matter as well as the overall geometry of the universe.
There is both theoretical as well as observational evidence to believe
that our universe is flat. But we are still not completely sure about what
constitutes our universe. The total amount of normal matter (baryonic matter)
provides only about 1% of the density needed for a flat universe. It is
widely believed that our universe is dominated by an unknown form of matter
called the Dark Matter which interacts weakly with other forms of
matter as well as light. Recent observations suggest that even this is
probably only a small part of the total amount required and the major part
is in the form of some unknown form of matter which has negative pressure
often termed as Dark Energy. With the future observations we will
probably obtain a clearer picture of our universe.
Penzias and Wilson (1965) discovered that our Universe is filled with a radiation at a temperature T=2.73 Kelvin. Due to its low temperature its peak intensity is in the microwave range. The energy spectrum of this radiation is now known to be black body Planckian spectrum to a very high accuracy. Where could this radiation have originated? We know that our local universe is fairly transparent to radiation, and under such circumstances it is not easy to explain the perfect black body nature of the radiation. To come into thermal equilibrium the radiation should spend a long time in close interaction with matter. This indicates that out Universe must have gone through a dense hot phase in its evolution. This is the standard hot Big Bang picture of the Universe. In other models of the Universe, for example The Steady State Model, the universe does not go through a hot dense phase and hence some other hitherto unknown process is required to produce this radiation. One such idea being that the light from the old stars is reprocessed by iron whiskers in the inter-galactic medium to produce CMB.
The Universe was dense and hot enough to promote nuclear reactions which built up the the chemical elements. In their seminal paper Alpher, Bethe and Gamow (also known as "alpha-beta-gamma" paper) showed that it is possible to cook up the lighter elements in the primeval soup if the temperatures and densities were high enough. The relative abundance of these elements turn out to be quite close to what is observed in the old stars which lends support to this idea.
This hot radiation was kept in thermal equilibrium by a soup of electrons
and protons when the Universe was still hot. As the Universe cooled down
due to the expansion of the Universe, the protons combined with the electrons
to form the Hydrogen atoms which decoupled themselves from the radiation.
This radiation was able to move freely through space and gradually cooled
down to a very small temperature. Its temperature is T= 2.73 Kelvin at
the present time. It almost perfect black body spectrum has deviations
of the order of ~ 10-5 % . These deviations from the perfect
black body provide clues about the fluctuations in the matter density of
the Universe at the time of decoupling and are used as seed perturbations
when we simulate the evolution of structures by the process of gravitaional
instability.
About half a million years after the Big Bang, the smooth fluid made of individual electrons and protons, that filled the Universe, combined to form hydrogen. This neutral and smooth matter distribution (with small inhomogeneities) collapsed due to gravitational instability and formed the first cosmic structures, which then evolved into stars and galaxies that lit up the Universe. Some time between redshift of 7 and 15 (when the universe was around $10^9$ years old), stars within protogalaxies created the first heavy elements. At these redshifts, the other systems which were luminous were the QSOs or the quasars. QSOs are objects that, at first glance, appear as normal stars. Upon closer inspection, however, QSOs have very large redshifts (i.e. the light they emit is strongly displaced toward the red end of the spectrum). Although their exact nature is controversial, they are commonly considered to be extremely distant, unusually bright nuclei of galaxies. The stars, together perhaps with an early population of quasars, generated the ultraviolet radiation (light whose frequency is just above the violet end of the visible spectrum) that reheated the cosmos and tore apart the hydrogen and other elements into electrons and protons again (called the ``reionization'').
The history of the Universe during and soon after these crucial formative stages is recorded in the all-pervading intergalactic medium (IGM), which is believed to contain most of the ordinary baryonic material (like protons and electrons) left over from the Big Bang. Throughout the various stages of structure formation, the IGM becomes clumpy and acquires peculiar motions under the influence of gravity. The gas in the IGM gets accreted, cools and forms stars within galaxies. Also, the same stars and galaxies dump the metal enriched material, energy and radiation ejected by them into the IGM. Because of these, the IGM is very rich in information on the formation of different cosmic structures.
The IGM is observed through absorption lines of distant quasars -- the
light coming from a distant quasar gets absorbed in the IGM, which can
be recognized through a telescope. These observations of absorption lines
provide invaluable insight into the chemical composition of the IGM and
primordial density fluctuation of some of the earliest formed cosmological
structures, as well as of the ultraviolet background radiation that ionizes
them.
When we look at far away galaxies we do not see them as they are now but as they were when the light reaching us today started its journey towards us. This is the only way we can study the history of the Universe, by looking far into the depths of space. To look deeper into space we need bigger telescopes to collect more light because distant objects are fainter. An astronomer is interested in measuring the amount and composition of light from the cosmic objects. She is also interested in the morphology of these objects. But distant objects are also small in their angular extent so we need telescopes with better angular resolution.
Turbulence in the Earth's atmosphere blurs the image of objects seen through the atmosphere. This appears as twinkling of the stars when seen through naked eye. Ground based telescopes are limited in their resolving power by the atmospheric turbulence. The best ground based telescopes can manage a resolution of about 1 arcsec. This is the angle subtended by a rupee coin at a distance of about 2 Km! For comparison our eye has a resolving power of about 1 arcminute (60 arcsec). The Moon subtends an angle of half a degree.
In addition to atmospheric turbulence observation from the ground are
also affected by local weather conditions (cloud formation, relative humidity,
temperature etc.) To get rid of the atmospheric turbulence astronomers
have built a telescope in space called Hubble Space Telescope (HST). This
telescope is not limited by the atmospheric turbulence and can give an
angular resolution of a fraction of an acrsecond. In addition it can observe
the same field of view for a long time since the observing conditions that
vary on the ground, do not change with time in space. Such images provide
us with invaluable information about the distant Universe.