Cosmology is a vast field, enveloping many possible research pursuits. This fact is partly because cosmology is the intersection of many seemingly disparate fields of physics. It covers a wide range of times, from the beginning of our universe to its different possible fates, and a wide range of length scales, from the chaotic Planck scale where space-time is ruled by quantum gravity to the size of the causally connected universe and beyond. There are many research and career avenues one can take in the field of cosmology.

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Probably the most recent and prominent endeavor making headlines is the work of the Laser Interferometer Gravitational Wave Detector (LIGO) and the Virgo interferometer. Thanks to the interferometers, which can detect a warp in space-time of the order of a proton radius, and the hard work of the collaborating scientists, gravitational waves that have travelled megaparsecs to Earth have been discovered. Based on the waveforms of these waves, the source of these waves can be inferred. Binary black holes produced the first five wave events, and the most recent sixth event originated from a binary neutron star system. This event was the first multi-messenger event, meaning that the gravitational wave signal was accompanied by an electromagnetic counterpart, which implied very strict limits on the speed of gravitational waves in gravity models, more precise localization of the binary neutron star system, and a deeper glimpse into the nuclear r-process involved in the kilonova that resulted from the merging of the binary system. The precise equation of state describing the contents of a neutron star is not known; perhaps there is a very dense quark-gluon plasma at its core, or perhaps the core is something else. It is hoped that this event and others like it will lend insight into this mystery. The inception of multi-messenger astronomy will also provide more accurate measurements of the rate of universal expansion, given by the Hubble parameter, since such measurements are independent of the cosmic distance ladder. (Our knowledge of distances to luminous objects in the sky is built up rung by rung on the cosmic distance ladder, starting from our knowledge of distances from closer objects and inferring distances, via potentially dubious assumptions, to farther objects such as supernovae.) Already, we have high-energy particle physics, nuclear physics, general relativity, and alternative theories of gravity coming together in this exciting work. Other proposed gravitational wave detectors, such as the Laser Interferometer Space Antenna (LISA) and the Einstein Telescope (ET), are currently in development, and determined experimentalists and theorists are needed to push these projects forward.

The cosmic microwave background (CMB) is another great source of cosmological data. When the universe cooled enough so that electrons and protons could form hydrogen atoms, thermal radiation was no longer being constantly bounced back and forth between the particles and could freely propagate away, and this was the first moment of electromagnetic transparency for our universe. This radiation left over from the Big Bang is fairly uniform over the sky, fitting a blackbody spectrum of temperature of about 3 Kelvin. However, there are small-scale anisotropies in the midst of the overall large-scale uniformity of the radiation, and the distribution of these anisotropies provides constraints on different cosmological parameters such as the Hubble parameter, the relative contributions of dark matter and dark energy to the universe’s energy density, and others. The Planck satellite and the Wilkinson Microwave Anisotropy Probe (WMAP) have made very precise measurements of the CMB and have provided strong observational constraints on these parameters. Before light decoupled from baryonic matter and propagated away to form the CMB, the counteracting forces due to inward gravitational attraction and outward radiation pressure caused baryonic acoustic oscillations, or BAO. Imprints of the characteristic size of these oscillations were made once radiation decoupled and formed the CMB, and these oscillations can be compared to their size today inferred statistically from galaxy surveys to constrain the expansion and acceleration of the universe over this time period in between. Another aspect that cosmologists study is the polarization of the CMB. The radiation can be polarized as either E-modes or B-modes. B-modes, which have vanishing divergence (much like the magnetic field in Maxwell’s equations), are produced by the gravitational lensing of E-modes, and the South Pole Telescope has measured such modes. Collaborations such as the Background Imaging of Cosmic Extragalactic Polarization (BICEP) and Keck Array also measure B-modes of the CMB. An important discovery that they hope to make in the future is the detection of B-modes produced by gravitational waves from cosmic inflation, which would be bolstering evidence for cosmic inflation, the supposed rapid expansion of the early universe that provided causal connectivity between distant parts of the CMB sky and a natural explanation for the observed spatial flatness of our universe.

In my opinion, one of the most fascinating aspects of our universe is that about 96% of it is comprised of stuff we do not really understand! About 28% of our universe is dark matter, and about 68% is dark energy.

Dark matter has only been detected gravitationally so far, and the candidates for dark matter include macroscopic objects, such as black holes and massive compact halo objects (MACHOs), and many non-baryonic particle models, including weakly interacting massive particle (WIMP) models. Dark matter was first inferred from the rotation curves of galaxies, which seemed to indicate that there must be some unseen mass providing the gravitational potential needed for the orbiting rates of stellar matter near the outer reaches of galaxies to be as high as observed. Direct detection experiments that look for direct interaction between dark matter and a target material have strongly constrained the allowed interaction cross section due to non-observation, and indirect detection may potentially come from the detection of decay products, such as neutrinos that the IceCube experiment may detect, or cosmic rays accelerated by supernovae that the AMS-02 experiment has studied. There is currently a 3.5-keV radiation signature coming from certain galaxies (and which is noticeably absent in others) that may be explained by interactions with dark matter.

Dark energy was first inferred from studying light spectra coming to us from Type Ia supernovae, and the way their emitted light was redshifted implied they were accelerating away from us. Other observational sources have confirmed the signatures of dark energy and dark matter as well. Dark energy could be due to a new field not included in the Standard Model of particle physics, or it could be vacuum energy due to a cosmological constant term in Einstein’s equation of general relativity. The measured value of dark energy as a cosmological constant and the calculated Standard Model vacuum energy are inconsistent by over 100 orders of magnitude! This discrepancy is known as the cosmological constant problem. The energy density of dark energy as a cosmological constant is constant as the universe expands, which is counterintuitive compared to the decreasing density of normal matter as the universe expands. Cosmological data also leave room for an exotic form of dark energy known as phantom dark energy, which is dark energy that causes an acceleration rate that increases over time. Thus, at some finite time in the future, the universal acceleration could be strong enough to rip apart bound structures, from galaxies to tiny atoms, and this fate of the universe is known as the big rip. Some of my research has concentrated on phantom dark energy, including the classification of alternative fates of the universe.

As we have seen, there are many avenues of research and various fascinating phenomena to study and observe in the vast field of cosmology, spanning from the earliest observable times to the possible end of the known universe. It integrates many subfields of physics that describe the smallest of particles to the largest of cosmic structures. We still have not covered all the purview of cosmology, including the study and simulation of cosmic structure formation, the detection of cosmic rays from blazars accompanied by neutrinos of unexplainably high energies, the complex baryonic dynamics and interactions in galaxies that are not fully understood, the effects of quantum gravity on density fluctuations in the early universe and at the center of a black hole, and much more. However, I hope this article has piqued your interest and given you a glimpse into an exciting field with many fulfilling research and career paths. All of the collaborations and experiments I have mentioned (along with others I left out) require dedicated theorists and experimentalists to advance the field further into our current era of precision cosmology. The future of the field is unknown, but it is sure to reveal even deeper and more beautiful mysteries.