Study of the Cosmos, at best, is considered a semi-scientific discipline, primarily because the la-boratory for carrying out measurements and tests of theories (the Cosmos) has been largely inac-cessible for centuries. The cosmic vista into the yonder, however, continued to fascinate humankind due to its inherent beauty and sheer curiosity. The invention of the optical telescope more than five centuries back, however, led to the opening of observational cosmology as a scientific discipline with firm experimental basis. However, the investigations based on visible light posed obvious limitations for the range of such observational cosmology. The advent of the radio telescope in the first half of the 20th century marked a fundamental new step in the progress of this branch of science. There has been no looking back in the march of knowledge in the discipline since then. A whole new vista was laid bare as a result of this development, leading to the discovery of altogether new celestial objects, such as quasars and pulsars and still newer galaxies. The parallel progress of the physics of fundamental constituents of the material world and their interactions led to an interesting merger of these two branches of physical sciences, yielding absolutely astounding knowledge of the nature and evolution of the Universe. New concepts of dark energy and dark matter thought to constitute the dominant share of the Universe were brought to light as a result of these new observations and theoretical ideas. This brief article aims to provide an overview of these exciting developments in the field of cosmology and the associated physics.
The birth and evolution of the Universe, of which we are one of the tiniest parts, have been an enigma and a challenge to unravel ever since humankind started taking a rational view of things around him. For a long time, Cosmology, as a branch of Physics, (Astrophysics) has been regarded more as soft science based on guesswork
and heuristic ideas devoid of any solid scientific base, largely due to absence of concrete observational evidence. However, as the means to explore the Universe came within man’s reach as a result of developments in various branches of physics and science, in general, more and more hard evidence on the different celestial objects and the universe beyond our immediate neighborhood in the cosmos started emerging and the revelation of extraordinary nature and properties of these objects fired the imagination of physicists and scientists, in general, to reach beyond our immediate galaxy, the Milky Way. A brief introduction to how it became possible to do so and with what scientific tools would be provided in the early part of this report. As amazing new facts about the nature of the astronomical bodies and systems inhabiting the cosmos emerged as a result of availability of these advanced means to access distances hitherto thought beyond our means to explore, a coherent picture of the origin and evolution of the universe started to emerge by the late 20th century, culminating in the so called Standard Model of Cosmology. This was greatly helped by exciting new developments in the realm of particle physics heralded by the advent of large and expensive accelerator machines, allowing exploration of the structure of the tiniest of the constituents of matter and hence of our universe. Although these developments led to a lot of clarity in our understanding of the science of cosmology, some new enigmas also emerged in their wake. Some of these unanswered questions will be touched upon towards the end of this article.
Our understanding of the cosmos has, in general, been impeded by the lack of experimental means to observe it. These observations were limited to the range of the optical telescope invented by Galileo in 1609, to begin with. However, this scientific development, momentous though it was, could only extend our knowledge of the cosmos (at whatever rudimentary level possible), not far beyond our own solar system or the Milky Way galaxy, in which it lies. Our observations and, therefore, directly measured data, which are the basis of any scientific understanding of a system, could, at most, cover a miniscule fraction of the universe as long as we were limited to the light (radiation) spanning only the visible part of the optical spectrum. Clearly, we had to extend our capability of observation to the use of ‘light’ beyond this tiny band of wavelengths, if we intended to explore the elements of the Universe well beyond, since it was established soon after the discovery of the electromagnetic waves, which carry information about their origin and whatever lies in between the observer and such sources, that the cosmos was full of electromagnetic wave signals, harking for research and investigation to establish their nature and origins, just as visible light gave us a clue of what we were looking at around us and our earth. The development of electronic technology and the sensitive tools to detect even weak signals around us first brought us to the advent of radio astronomy. This made it possible for mankind to extend his sphere of exploration of the Cosmos to the ‘invisible’ (to the human eye or the optical telescope) part of the universe.
Radio astronomy started with a serendipitous discovery in 1932 by Jansky [
Planetary science of our solar system benefitted from the development of the radio telescope greatly, since, although planets only reflect visible light, they may, however, emit radio waves, leading to their detailed studies
using the new type of telescopes. The surface temperature of Venus was measured from such observations [
development of radio astronomy, thus, expanded the horizon of our knowledge and understanding of the Universe from the elements of our own solar system and its planets to stars and other objects in our galaxy, to interstellar space and galaxies far beyond.
The discovery of cosmic microwave background (CMB) radiation in 1964 [
Whereas the uniformity of CMB in all directions in the cosmos seemed to be a striking feature of its discovery, detailed later studies based on COBE (Cosmic Background Explorer), a satellite in the Explorer series launched by NASA on November 18, 1989 in a sun-synchronous orbit, revealed strong evidence for an all important anisotropy in CMB, first announced on April 23, 1992 [
The COBE mission was followed by the Wilkinson Microwave Anisotropy Probe (WMAP) launched on June 30, 2001 with the objective of obtaining a more precise full sky map of the anisotropy of CMB distribution with a resolution of 13 arcminute - 45 times more sensitive and with 33 times the angular resolution of COBE. These precise data were expected to help understand the geometry, content, and evolution of the Universe, providing finer tests of the Big Bang model. WMAP’s measurements played the key role in establishing the current Standard Model of cosmology and have led to the most precise value, till then, of the age of the Universe at 13.75 ± 0.11 billion years; the full timeline of the Universe according to the standard model is given in
This insight into the nature of the Universe has led to two remarkable new concepts. The first is to do with the expansion of the Universe referred to above. It has all along been expected that the speed of this expansion would slow down as we move farther and farther out in the Universe, since the galaxies would gravitationally pull each other. It was, therefore, to every body’s astonishment that, to the contrary, this expansion was found to be accelerating rather than slowing down as we move outwards. This amazing recent discovery based on the observations of the distant Type 1a Supernovae―a late-in-life, dying state of a star―in 1998 [
Another subject of immense current research is an equally, if not more, mysterious object called the “dark matter”. This is matter proposed to provide the gravitational glue binding the galaxies together in the cosmos. The original idea for this type of matter arose from the fact that galaxies in some clusters are found to move too fast to be allowed to hold together among themselves; even some stars within some individual galaxies move way too fast for gravity to hold them in these galaxies. Some mysterious invisible “dark matter” was, therefore, proposed to provide the missing gravitational pull in these systems, as early as 1933 [
The European Space Agency’s (ESA) Planck mission, launched to study the cosmic microwave background (CMB), released the first results after the initial 15 months in March 2013 [
With the initial, opaque plasma state of the Universe after the Big Bang, the excessive energy present did not allow formation of stable atoms until a cooling period of 380,000 years, when an event known as recombination set in. The transparent universe followed with a huge outflow of photons, into the modern era. The initial ultraviolet photons resulting from recombination, however, converted to the microwave spectral region as the universe cooled further down to its current level. The power spectrum of the fluctuations over the sky, revealed by
Planck mission,
The ultimate evidence for or against dark matter is expected to emerge from the polarization studies of the CMB radiation over the Universe and its anisotropy signature. A great excitement was recently generated by positive news on that count, coming from a telescope station located at the South Pole of the earth. On March 17, 2014 came the announcement of the observation of the so called primordial B-modes by BICEP2 detector of the telescope. These are swirling polarization patterns,
However, it did not take too long to realize that the twisting CMB polarization pattern could easily be caused by the cosmic dust in the Milky Way galaxy [
This reversal of the evidence for the detection of primordial gravitational waves applies brakes to the progress in the quest for experimental proof of Big Bang inflationary model, to the idea of multiverse and, of course, to a definitive proof of the existence of dark matter. However, this may signal a temporary hiatus, since, at least, eight experiments, including BICEP3, the Keck Array and Planck, are already focused on this problem. Mean-while, a new perception has emerged regarding the inflationary model, which makes this theoretical model valid irrespective of the experimental evidence for the primordial gravitational waves―the paradigm of inflation would, in that perspective, appear to be unfalsifiable and, therefore, scientifically meaningless [
The brief review above shows clearly how far we have travelled in unraveling the secrets of the Cosmos and the Universe in which we live, over less than a century. It is equally manifest that this rapid progress has been made possible by the enormous advancements in the experimental instrumentation and techniques available for observing the Cosmos. The huge progress in telescopy and the remarkable developments in space technology have made a tremendous impact on the achievable knowledge with high precision and accuracy. While the astronomical knowledge of the cosmos has witnessed impressive progress, the analysis of these observations has opened new frontiers in our understanding of the fundamental physics and nature and evolution of the Universe.
It is clear from the above brief description of the current state of our knowledge of the Universe that the age old maxim, the more we know, less we conclude we actually know, appears to be perfectly valid. However, such
is perhaps the nature of all human knowledge. But how exciting the ever expanding frontiers of this field of fundamental physics are can be gauged from this brief narrative. It is also clear that our quest for the knowledge of the Universe will be a continuing chapter in the story of human search and discovery for the years to come and may yet raise more questions than it can answer. Thanks to the remarkable developments in technology, and hence the tools of observational astronomy, many new frontiers of the unknown may yet be opened for even more extensive exploration of the Cosmos than the territory conquered hitherto.