Our Cosmic Habitat

Today’s Physics post is about modern cosmology–the composition and behavior of the universe at the grandest scale, and an introduction to the “fine tuning problem.” The second paragraph is a recap from the last physics post, “Echoes of the Big Bang.” The appendix lists five of the many techniques for measuring distance in the cosmos. Enjoy!

​Martin Rees’s Our Cosmic Habitat outlines basic cosmology, star and element formation, astrobiology, and some of the more modern ideas floating around in Physics. Rees provides a gentle introduction to the core concepts of cosmology and underscores an important philosophical issue that arises from the current understanding of the history and status of the universe; how can one explain the several seemingly “fine-tuned” aspects of fundamental physics? This is a formidable philosophical issue since there seem to be a number of seemingly independent phenomena which, if they were only “slightly” different, would result in a universe much more hostile to life than our universe already seems to be. Rees cites several examples: If gravity were a bit stronger, then stars would be small and burn out quickly; if gravity were a bit weaker, then stars, galaxies, and higher elements necessary for life wouldn’t form at all; if the “cosmic texture” constant Q were a bit larger than the current value of 0.00001, then the universe would have been so lumpy early on that most stars would have formed as giant black holes; if the nuclear force were a bit weaker in comparison to the electromagnetic force, then the only stable element would be hydrogen, which would be quite problematic for life as is currently known; if the nuclear force were a bit stronger in comparison to the electromagnetic force, then elements would form so easily from hydrogen that stars would evolve so differently that it is hard to measure the impact on life; and finally, if the carbon molecule that is so vital to biology on Earth were to have a vibrational mode of slightly different energy, then it would be so rare as to stifle the evolution of life. While Rees provides an accessible introduction to many aspects of cosmology, his most important contribution to the ongoing discussion that is Physics 361 is his outline of the Fine-Tuning problem. As far as Big Bang Cosmology in particular, the previous book entitled Echo of the Big Bang by Michael Lemonick provides a superior account of the evidence for the current understanding of the history of the universe, its composition, curvature, and overall structure. The fact that Rees’s book was published before the findings of WMAP make Our Cosmic Habitat somewhat irrelevant as a source for the evidence of standard Big Bang cosmology. It is for this reason that I will borrow heavily from discussions from class, as well as the previous paper about Lemonick’s book in order to summarize modern cosmology.

​The simple observation that the farther away a galaxy is from Earth, the faster the galaxy seems to be moving away implies that space itself is expanding in all directions. This expansion has always been happening, so the universe must have been much smaller in the past. Theoretically, if one were to rewind the cosmic clock, the universe used to be a single point, otherwise called a singularity. The universe probably was not a singularity, since a singularity doesn’t make any physical sense, but the model works for the time being. Playing the cosmic clock in the forward direction from the time of that singularity, one notices that the entire universe is very dense, and thus very hot. Theoreticians like Gamow, Alpher, and Herman noticed that a hot, dense universe might emit a Blackbody spectrum, but the dense soup of other particles would block the emission until the universe were sparse enough to allow atoms to form—which turned out to be about 380,000 after the singularity began expanding. As the universe expanded, the Blackbody emission would be redshifted. Gamow calculated that the emission should be detectible as microwaves. Furthermore, since the universe was uniformly hot in those early times, these microwave emissions should be uniform throughout the sky. There may be various ways to explain the observation that more distant galaxies move faster, but the existence of a ubiquitous shower of microwaves coming from all directions from a distance that corresponds to the time that the Blackbody emissions supposedly broke free confirm beyond a doubt that the universe began quite small and expanded to become the current universe. More subtly, the small perturbations in a mostly uniform CMB also give hints as to the physics of the very early universe. These small perturbations are important since they shaped how the galaxies formed and determined how matter is currently distributed throughout the cosmos. A completely homogeneous CMB would lend itself to a uniform distribution of galaxies, but our universe has an intricate web-like structure that is well explained by the aforementioned small anisotropies in the CMB. Some additional pieces of critical information can be gleaned from the so-called “acoustic peaks” observed in the CMB. These peaks give information as to the composition of the universe—the proportions of dark matter and baryonic matter—as well as its curvature.

​The WMAP satellite provides the most comprehensive audit of the CMB to date. However, it is important to note that many of the findings of WMAP are corroborated by several ground-based experiments, as well as unrelated astronomical observations. The data from WMAP has allowed scientists to scrutinize the small perturbations in the CMB in order to gain insight into structure, composition, and geometry of the universe. These slight heterogeneities, or “anisotropies,” in the CMB provide clues as to how regular atoms, or “baryonic matter,” interacted with so-called “dark matter” in the early universe. This allows scientists to establish the composition of all the mass-energy of the universe. The mass-energy of the universe can be divided into four parts: Neutrinos—a nearly massless particle that interacts very weakly with atoms—account for 0.4% of the universe; regular atoms, or baryonic matter, account for 4.6% of the universe; dark matter accounts for 23% of the universe; and finally, dark energy accounts for 72% of the universe. So, it seems that all of the matter for which physicists have toiled and struggled so much in order to explain—namely, atoms and subatomic particles—comprises a measly 5% of the universe. So, what is the other 95%? What is dark energy, and what is dark matter?

​It does not serve justice to the shear befuddlement of modern Physics to simply say that we do not know what dark matter and dark energy are. That would be too easy of a response. Each of the two provides a distinct and troubling level of profound confusion. Dark matter was first discovered in 1933 by astrophysicist Fritz Zwicky. Basically, if there were no dark matter, then there would not be enough mass in galaxies to explain their gross behavior. One might think that there is some theoretical misunderstanding that could eliminate the need for dark matter, but in recent years, several lines of observational experiments lead to the conclusion that dark matter is real, physical “stuff.” One line of experiment comes from the observation that the giant cloud of hydrogen that circles any given galaxy is moving too quickly to be explained by the presence of the ordinary matter in the galaxy itself. Worse yet, even clusters of galaxies could not hold themselves together with ordinary matter alone. These calculations can be done with Newtonian Mechanics with little backlash from relativistic effects. Again, one might suggest that perhaps Newtonian physics does not hold on scales larger than the solar system or the Milky Way. Unfortunately, a completely independent line of observation based on Einstein’s General Relativity confirms the same conclusion. In Einstein’s view of the universe, mass bends space and stretches time, and as a result, mass can bend the path of light. This effect is called “gravitational lensing.” There is not nearly enough ordinary matter in observed galaxies to explain the amount of gravitational lensing that is observed. Both the Newtonian and Einsteinian perspectives lead to the same result—there is much more matter in the universe other than the comfortable light-emitting stars astronomers know and love. So, physicists are forced to accept the presence of a cold, dark, mysterious form of matter that pervades the universe in vast quantities. What is even worse is that this matter dominates baryonic matter by five fold. Not only does dark matter exist; it is the majority of matter. There is hope yet, since some day soon scientists might determine dark matter to be the very dim Brown Dwarf stars, supermassive black holes, or perhaps a yet undiscovered particle.

​While light may soon shine on dark matter, the situation seems much more bleak for dark energy. As much dark matter as there is, there is over three times as much dark energy. Unlike dark matter, it is not clear that dark energy is actually “stuff.” In fact, dark energy is likely not “stuff.” Dark energy is a name for the energy that fuels the expansion of the universe. Astronomical observations have established that the universe is not only expanding, but also accelerating rapidly in that expansion. This simply doesn’t make any sense. In order to explain the accelerating expansion of the universe, theoreticians currently have two options. These two options are to posit a mysterious dark energy to fuel the expansion, or to invoke the long abandoned “cosmological constant” from Einstein’s General Relativity. Adding a dark energy to the universe is troublesome because any given amount of dark energy would become less dense as the universe expands, and thus dark energy would become less and less effective at fueling expansion over time. This pattern is not observed. The acceleration of the expansion is observed to be constant, not decreasing as the density of some mysterious dark energy decreases. So, a better model seems to be the cosmological constant, denoted by the capital Greek letter lambda. Einstein first invoked the cosmological constant as a mysterious force to perfectly counter gravity, resulting in a steady universe that is neither expanding nor contracting. Now, the cosmological constant is needed to describe a universe that is expanding out of control. So, between dark energy and the cosmological constant, it seems that a cosmological constant is more consistent with observation. This is wholly unsettling. A mysterious dark energy would at least provide a physical explanation for the expansion of the universe, obscure as it may be. A cosmological constant is an arbitrary constant of integration in Einstein’s equations with no known physical description. PAM Dirac first suggested that the cosmological constant might just be the result of Newton’s gravitational constant G not actually being a constant, but rather a parameter that decreases with the age of the universe. In this way, expansion could be fueled by the fact that gravity is getting weaker and weaker. In light of the deeply troubling existence of dark matter and the nonsensical existence of a cosmological constant, the triumph of WMAP in discovering the history and structure of the universe seems only a small consolation.

​Big Bang cosmology has survived over thirty years of rigorous experiment and observation. The theory could have been broken several times, but it endured with fantastic precision. Incredible feats of engineering have provided the direct evidence necessary to describe the history of the universe from 380,000 after the beginning of time, as well as the indirect evidence to describe the adolescence of the universe from less than one hundred millionth of a second of the Bang. With that being said, there are still profound mysteries in the universe. Dark matter and dark energy could be the tip of an iceberg as profound as the induction of Quantum Mechanics in the early twentieth century.

Appendix:
1. Parallax: the angular positions of a star measured from two different positions in the Earth’s orbit around the Sun combined with the distance between the two observation points can be used to determine the distance of a star using trigonometry. The range of this method is a few hundred parsecs.
2. Standard Candles: Variable stars have known luminosity that is dependent on the period of pulsation. Since absolute luminosity is known, the brightness of a variable star indicates its distance.
3. Tully-Fischer Relation: An empirical relation between the luminosity of a spiral galaxy and the width of observed spectral lines. This method is useful to 50 Mpc.
4. IA Supernovae: these supernovae occur when a binary white dwarf star begins to accrete matter from its companion Red Dwarf star. The process leading up to an IA event is known, and so distance can be measured as with standard candles. This method is useful to over 100 Mpc.
5. Planetary Nebulae: An empirical relationship between the luminosity of a certain type of nebula and the luminosity of the galaxy it is in. The luminosity of certain dying stars in the nebula is a constant multiple of the average luminosity of the galaxy. This method is useful at around 12 Mpc.

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