On March 17, the BICEP2 collaboration that made exquisite measurements of the Cosmic Microwave Background (CMB) from the south pole over three years, with a state of the art detector array, announced the detection of a distinct signature of cosmic gravitational waves, possibly originating during an explosive phase of expansion dubbed inflation, occurring at the earliest moments after the big bang. BICEP2 belongs to a novel series of ground-based microwave telescopes at the South Pole designed to exclusively target the feeble 100 nano-Kelvin B-mode polarization signal in the CMB sky expected to arise due to primordial gravitational waves. The observing site of BICEP2 is dictated by the lack of moisture in the atmosphere that could obscure the microwave photons. BICEP2 marks a significant technological improvement in sensitivity and breaks away from the standard mould of CMB experiments with a design closer to the conventional optical telescopes than radio antennae.
Through the paradigm of inflation, the BICEP2 observations provide us with a window to physics at very high energies, a thousand billion times higher than the energies that are currently achieved by the Large Hadron Collider at CERN. Interestingly, the inferred energy scale of inflation — of the order of 10 million billion Giga electron Volts (GeV) — is close to that expected in the grand unified theories of three of the four fundamental forces. These energies are unlikely to be explored by even the most monstrous man-made particle colliders that can be set up in the conceivable future. In this article, divided into two parts, we shall first place in a wider context the observations made by BICEP2 and then discuss the implications of the observations for the physics of the early universe.
At the end of the article, under the FAQs, we have made an effort to clarify separately some of the questions that a reader may have in as simple a manner as possible.
The observations by BICEP2
The main aim of cosmology today is to comprehend the origin and evolution of the universe, which, in the currently standard picture, is linked to understanding the composition and distribution of matter in the present universe.
In order to do so, it has been useful in cosmology to think of the universe in terms of the origin and evolution of tiny fluctuations on a smooth background spacetime. This has allowed, over the years, the community to make very reliable theoretical predictions that can be tested against the increasingly refined cosmological observations that have been possible in the past two decades. The composition and evolution of the smooth part is described by the popular hot Big Bang model. According to the model, the universe began in a hot dense phase, with its expansion being driven by radiation. It cooled down as it expanded, and transited to an epoch dominated by matter. Theoretical modeling of the formation of light elements in the early universe coupled to observations point to the fact that only about 5 per cent of the universe consists of ordinary matter (referred to by cosmologists as the baryons). Other observations, including those involving distant supernovae, suggest that the rest of the matter in the universe today comprises of 25 per cent of non-relativistic dark matter and about 70 per cent of dark energy. In fact, the dark energy being dominant today implies that the
universe is presently going through an accelerated rate of expansion.
An important pillar supporting the hot Big Bang model is the detection of the CMB. The energy in the Cosmic Microwave Background is insignificant today, but it is a vestige of the early epochs when the energy density of radiation dominated the expansion of the universe. The CMB (originally serendipitously discovered by Arno Penzias and Robert Wilson in 1965) is nearly, perfectly thermal (corresponding to a temperature of about 2.73 K) and it is extraordinarily isotropic: its temperature is the same in all directions of the sky up to 10 parts in a million. The Cosmic Microwave Background arises due to the fact that radiation ceased to interact with matter when the universe was rather young (about 400,000 years old, while the current age is about 14 billion years), an epoch known as decoupling. The CMB photons travel to us from this surface of last scattering, over 42 billion light years, virtually unimpeded, across the universe.
The fact that the CMB is exceedingly isotropic implies that matter was distributed smoothly to a high degree during the early stages of the expansion. In fact, it is this observation, together with observations of the distributions of the galaxies on very large scales, that provides the basis for theoretically dividing the universe into a smooth and an inhomogeneous part, and studying their evolution independently. Though exceedingly isotropic, the Background contains small anisotropies of about 10 parts per million. These anisotropies were first detected by COBE in 1992, and various Earth-based (QUaD, ACBAR and BICEP, etc.), balloon-borne (such as Boomerang, Archeops and Maxima) and space-based missions (WMAP and Planck) have observed these anisotropies with ever increasing precision over the last decade and a half.
The anisotropies in the CMB reflect the extent of the inhomogeneities in the universe at the time of decoupling. The anisotropies in the Cosmic Microwave Background are, in turn, signatures of the perturbations (i.e. the inhomogeneities) in the early universe, and their observations provide us with direct clues to the characteristics of the primordial perturbations. We should add that it is the anisotropies seen in the Background which are amplified by gravitational instability into the galaxies and clusters of galaxies that we see around us today. Anyway, these perturbations can be divided into two classes as scalars and tensors (there is, in fact, another type of perturbation called the vectors, which are not very relevant for our story), with the scalars representing matter perturbations, while the tensors correspond to gravitational waves predicted by Einstein's General Theory of Relativity, that cosmologists often work with to describe relativistic gravity.
The CMB, as it consists of photons, can be polarized as well. One finds that, the small extent of anisotropies already present, when scattered by the electrons at the surface of last scattering (as the Cosmic Microwave Background decouples from matter), leads to the emerging photons being linearly polarized. Linear polarization, as is well-known, consists of two degrees of freedom or modes. Mathematically, one can express these two modes in terms of what is called a scalar mode E and a pseudo-scalar mode B. Physically, such a decomposition of the CMB polarization turns out to be important as the scalar and the tensor perturbations leave distinct signatures on these modes. For instance, the B-modes are affected only by the primordial gravitational waves (on large angular scales, see below for a further clarification).
The CMB anisotropies are conveniently characterized in terms of the corresponding angular power spectra, viz. the power in the anisotropies at different angular scales or, equivalently, the so-called multipole moments. The BICEP2 experiment was designed to observe the angular power spectrum of the CMB in the B-mode of polarization, focusing on relatively large angles in the sky. Actually, apart from the primordial gravitational waves, there is another mechanism that can generate the B-modes in the Background. In General Relativity, the propagation of photons can be shown to be affected by gravitational fields, a phenomenon known as gravitational lensing. As the CMB photons travel towards us from the surface of last scattering, the gravitational lensing due to weak fields by the intervening matter are known to convert the E-modes of polarization into the B-modes. But, since this physics occurs over relatively small length scales, weak gravitational lensing affects only the smaller angles in the sky, i.e. over the larger multipoles.
On March 17, BICEP2 announced the detection of the angular power spectrum of the B-mode polarization of the CMB, parts of which are distinct imprints of primordial gravitational waves, and some other parts being the contributions due to weak gravitational lensing. An important conclusion of BICEP2 observations is that the amplitude of the perturbations in the tensors constitute 20 per cent of the total primordial perturbations (this result is quoted as the tensor-to-scalar ratio being 0.2), which, as we shall discuss in the second part of the article, has rather important implications for the physics of the early universe; in particular, the inflationary paradigm.
Before we conclude this part, we ought to clarify a couple of points. First, the observation by BICEP2 (assuming, of course, that the results are confirmed by other experiments) is an indirect detection of gravitational waves. A direct detection would correspond to an observation made by interferometers such as the Laser Interferometer Gravitational-Wave Observatory (LIGO). Second, we believe it is worthwhile mentioning here that a LIGO-India is being planned, which can play an important role in the direct detection of gravitational waves.
Implications for the physics of the early universe
The hot Big Bang model, though rather successful, has a major drawback. According to the model, two regions of the sky on the surface of last scattering which are separated by more than one degree in the sky today could not have interacted by the time of decoupling. In other words, the hot Big Bang model is unable to provide a causal mechanism to explain the extent of isotropy of the CMB. Yet, we find that the CMB from even anti-podal points on the sky that are separated by about 84 billion light years have almost the same temperature.
This drawback, called the horizon problem, is usually overcome by invoking a brief period of accelerated expansion during the very early stages of the radiation-dominated epoch. Such a scenario is called the inflationary paradigm. However, accelerated expansion of the universe is impossible to achieve with ordinary matter or radiation. At the same time, it can be easily achieved with the help of scalar fields, which are ubiquitous in theoretical high-energy physics. While inflation was originally proposed to overcome the horizon problem and explain the extent of isotropy in the CMB, in the modern context, the most attractive aspect of the paradigm is its ability to provide a simple and causal mechanism to explain the generation of perturbations in the early universe. It is the quantum fluctuation associated with the scalar fields that sow the seeds of the perturbations, which leave their imprints as anisotropies in the CMB. Therefore, through the paradigm of inflation, increasingly accurate measurements of the anisotropies provide us with an unprecedented window to physics at very high energies.
The scalar field that drives inflation is often conveniently referred to as the inflaton. One finds that even the simplest of scalar field models can act as the inflaton. This efficiency of the inflationary paradigm involving scalar fields also seems to be responsible for its major drawback. Theoretically, there exist way too many models of inflation, many of which had performed reasonably well against the CMB data from missions such as WMAP and Planck. The BICEP2 results can change the situation drastically for the following simple reason. It can be shown that, in most of the inflationary models, the amplitude of tensors generated by the model depends simply on the energy scale at which inflation occurs in the model. If the imprints of gravitational waves as seen by BICEP2 in the B-mode were indeed generated by inflation, it implies that the observed tensor-to-scalar ratio of 0.2 corresponds to inflation occurring at the scale of about 10^16 (i.e., 10 million billion) GeV, an energy scale which, as we mentioned above, is unlikely to be reached by even monstrous colliders.
We should add a couple of cautionary remarks before we wind up. There exist scenarios other than inflation that can generate perturbations in the early universe. Another competing scenario would be the less popular bouncing model, which can also provide a causal mechanism for the origin of perturbations. It is also well known that primordial magnetic fields and topological defects, the latter forming when symmetries break in the early universe, can give rise to scalar and tensor perturbations. It has been conclusively established that the contributions of the topological defects to the imprints of the scalar perturbations on the CMB are rather limited. Investigations seem to suggest that the contributions of the defects to the imprints of the tensor perturbations are limited as well. Between the inflationary and bouncing scenarios, the former seems more efficient and more attractive theoretically. We will leave it for debate if the BICEP2 data has conclusively established inflation. But it seems beyond doubt that further observations of the B-mode polarization of the CMB will aid us arrive at rather strong constraints on competing inflationary models.
(Dr. L. Sriramkumar is an Associate Professor at the Department of Physics, IIT Madras and Dr. Tarun Souradeep is a Professor at the Inter-University Centre for Astronomy and Astrophysics, Pune. Their current research interests are primarily focused on investigating issues in cosmology related to inflation and the Cosmic Microwave Background.)
1. What is the Big Bang?
It is the moment when the universe is supposed to have begun expanding. However, we should hasten to add that we do not yet have a good understanding of moments very close to the Big Bang. Note that General Relativity, which is the relativistic theory of gravity that is often used to describe the universe, is expected to fail close to the Big Bang, and a quantum gravitational theory (such as string theory or loop quantum gravity) is believed to take over. Investigations based on such theories seem to suggest that the Big Bang may be avoided, but the complete picture is yet to be arrived at.
2. What are radiation and matter dominated epochs?
The universe evidently consists of radiation and matter. According to General Relativity, these form the source of expansion of the universe. Based on their evolution as the universe expands, one finds that the energy density of radiation is dominant during the early stages, while the energy density in non-relativistic matter dominated at later times. These are referred to as the radiation and matter dominated epochs.
3. What are dark matter and dark energy?
Various observations point to the fact that apart from baryons (i.e., ordinary matter), which interact with light and hence can be ‘seen’, there seem to exist two other forms of matter in the universe, viz. cold dark matter and dark energy. The adjective ‘dark’ is used to imply that they do not interact with light. Cold dark matter behaves essentially like the baryons, while dark energy, in some sense, behaves as scalar fields do during inflation.
4. What is the Cosmic Microwave Background?
It is a perfectly thermal (i.e., it follows the famous Planck distribution to a great extent) and nearly isotropic (i.e., the same in all directions) radiation, which is a remnant of the early radiation dominated epoch and hence is ‘cosmic’ in origin. Though the contribution of the CMB to the total energy density of the universe today is negligible, it contributes the most to the energy density in radiation today.
5. What is the epoch of decoupling and what is the surface of last scattering?
In the early stages of the radiation dominated epoch, the photons interact with the electrons very strongly through what is known as Compton scattering. This strongly couples the radiation to matter. However, as the universe cools due to the expansion, the interaction rate between the photons and the electrons fall below the rate of expansion, as a result of which the photons largely stop interacting with matter. This phenomenon is known as decoupling. Thereafter, the photons travel virtually uninterrupted until they reach us today. The spherical surface (as we look at the sky) from which these photons emerge wherein they were scattered last by the electrons is known as the surface of last scattering.
6. What are anisotropies in the CMB?
While the CMB is largely isotropic, i.e. carrying largely the same temperature in all directions of the sky, it contains tiny (of about 10 parts in a million) deviations from isotropy, which are referred to as anisotropies.
7. What are inhomogeneities or perturbations?
Inhomogeneities are deviations from smoothness or homogeneity (i.e., the same at all locations). Small deviations from homogeneity are mathematically referred to as perturbations. The fact that the anisotropies in the CMB are small (10 parts in a million, as we mentioned above) implies that the inhomogeneities were small in the early universe as well. Hence, the inhomogeneities in the early universe are treated as perturbations, which, in fact, allows the problem to be mathematically tractable. We should add that the inhomogeneities are not small today, as can be seen in the galaxies and clusters of galaxies that we see around us. These inhomogeneities arose due to the gravitational instability (essentially, conventional Newtonian gravity that makes objects collapse on themselves) to form the large scale structures today.
8. What are scalar perturbations?
Perturbations, as we said, are small deviations from homogeneity. A scalar perturbation is a quantity that does not possess a direction. Examples of scalar perturbations are small deviations from homogeneity in the energy density, pressure in matter, or radiation.
9. What are gravitational waves, and why are they referred to as the tensor perturbations?
Gravitational waves are small deviations of a given spacetime, which propagate, like electromagnetic waves, at the speed of light. As in the case of electromagnetic waves, they can exist even in the absence of sources. As you may know, electromagnetic waves are vectors, characterized by the direction of their polarization. In relativistic theories of gravity, spacetime is described by a metric, which is a tensorial quantity (i.e., in contrast to a vector which contains only one index, say, representing the Cartesian components, a tensor is an object that possesses more than one index). As gravitational waves are small deviations of the metric in a given spacetime, they are referred to as the tensor perturbations.
10. What are the E- and B-modes of polarization of the CMB?
The CMB photons emerge from the surface of last scattering after having interacted with the electrons through the process of Compton scattering. This process leads to the CMB photons that we detect today to be linearly polarized. Linear polarization of an electromagnetic wave can be described in terms of the two, unchanging components of the electric field associated with the wave in the plane perpendicular to its direction of propagation. The E- and B-modes are simply a convenient way of decomposing the two-dimensional vector into its two components. The decomposition helps in isolating the effects of primordial gravitational waves on the CMB. As we have discussed, the gravitational waves affect only the B-mode polarization of the CMB.
12. What is gravitational lensing and why does it occur?
Gravitational lensing is the bending of light due to a gravitational field. In a relativistic theory of gravity such as General Relativity, this essentially arises due to the fact that light carries energy, and all sources of energy influence, and are influenced by, gravitational fields.
13. Why does weak gravitational lensing create the B-modes?
Weak gravitational lensing corresponds to situations wherein gravity is not strong. In such a case, gravitational lensing does not alter the intensity of the electromagnetic radiation appreciably, but simply shears the images. Such a shearing converts the E-modes (which are mathematically similar to, say, radially outgoing electric field distributions of a positive charge) into the B-modes (that are similar to curling magnetic field lines from a bar magnet).
14. What is inflation and why is it required?
Inflation corresponds to a brief period of accelerated expansion of the universe during the early stages of the radiation-dominated epoch. It is the most attractive scenario to overcome the horizon problem, which we have described in the article.
15. Do the BICEP2 results confirm inflation?
Not necessarily so. But if the BICEP2 results are corroborated, then it is an unambiguous signal of primordial gravitational waves. Among the different possibilities that could have generated them in the early universe, particularly at the strength observed by BICEP2, inflation seems to be the most promising candidate.