March 28, 2020

The Universe: You Are Here in Time & Space.

Our present understanding of our expanding universe. (Credit: NASA/WMAP).

(Editor’s Note: The essay that follows is a re-bloggified version of an essay I wrote in our quest for a science teaching degree. As that quest for knowledge has changed into a quest for employment, I thought it would be a worthy exercise to place these works out where eyeballs might fall upon them once again…)

Cosmology is one of the fastest evolving fields in astronomy today. In less than a century, our understanding of the past and future evolution of our universe has gone from one largely of conjecture to a diverse study with hard observational data. This current understanding was hard won, and new discoveries continue almost daily. Still, just how do we know our current situation in time and space, and what does the future of the universe hold?

The Greeks could be said to have been the first to have attempted to contemplate the scale of the universe without evoking supernatural means. Even then, there was no reason not to suppose that the area surrounding Mediterranean was the extent of the physical world, and perhaps the Sun, Moon and stars lay only scant miles overhead.

The slow existence of the acceptance of a heliocentric universe led to the first inkling of an idea of the scale of the solar system. The stars were assumed to be some distance away, because they showed no resolvable parallax. Measurements during the 1769 transit of the planet Venus over the disk of the Sun gave us the key to the scale of our solar system; However, the first stellar parallax wasn’t obtained until 1838. Astronomer William Herschel attempted to conduct the first “census” of nearby stars, and tried to deduce, with some success, the elliptical structure of our own Milky Way.

The discovery of the period/luminosity relationship for a type of stars known as Cepheid variables provided a “standard candle” to measure distances on a galactic scale. These stars also played a key role in what was perhaps the biggest breakthrough in modern times. In the 1920’s, American astronomer Edwin Hubble spied one of these key stars in the Andromeda galaxy and was able to use it to accurately measure its stupendous distance. It was soon realized that the multitude of spiral and irregular smudges seen throughout the sky were individual galaxies, some dwarfing our own Milky Way!

During this time, two leading theories of the origin of the universe vied for acceptance. The first, called the Steady State theory, was championed by Fred Hoyle, and posited that the universe had always existed and somehow renewed itself in a state of balance. The second, dubbed the Big Bang theory by Georges Lemaitre, theorized that the universe had a finite beginning, and had been expanding from that moment in time and space. The Big Bang gained large scale acceptance through two initial observational proofs: 1. the discovery of cosmic spectral red shift known as the Hubble Constant, predicted to occur if the universe were expanding; 2. the discovery by Penzias and Wilson of the cosmic microwave background in 1964, a relic of the initial Big Bang.   
But the universe hasn’t given up its secrets easily. In just the past few decades, we’ve acquired the tools and methods to test our predictions in regards to cosmology.

One new and powerful standard candle utilized by astronomers are known as Type Ia supernovae. These are a subclass of supernova resulting from a binary system composed of a degenerate white dwarf star accreting material from a bloated red giant companion. When the white dwarf reaches a critical limit, a massive explosion results that can be seen from across the universe. Like Cepheids, these explosions have an inherent intrinsic brightness, and if the galaxy containing the Type Ia can be pinpointed, its distance can be inferred.

The overall age of the universe is derived from the measurement of the Hubble constant, currently estimated at 70 kilometers per second per mega parsec. Thus, the estimated age of the universe since expansion began is estimated to be about 13.7 billion years old. Some of the oldest objects we see in the sky are termed globular clusters, which are ancient conglomerations of metal poor, Population II stars. In fact, these structures posed a major problem to cosmologists in the 1990s as they appeared to be older than the universe they inhabited! Refinements in techniques have now resolved this apparent paradox, and these structures now place a lower constraint of 12 billion years on the age of the universe. The search is also on for theorized population III type stars, which would have been the first early stars which fused exclusively hydrogen and helium. Our Sun is a later generation, metal rich population I star, and this is thought to be key the formation of rocky worlds such as the Earth and perhaps, the origin of life itself out of the same proto-solar nebula.

As observational techniques have progressed, it has also become apparent that the cosmic microwave background is not uniform, but has minute fluctuations in its density. Scientific platforms such as COBE, the COsmic microwave Background Explorer, and WMAP, the Wilkinson Microwave Anisotropy Probe have mapped the cosmic microwave background radiation, and this “clumping” is thought to have seeded the first galaxies. Other cutting edge observatories such as LIGO, the Laser Interferometry Gravitational wave Observatory are on the hunt for gravity waves and have likewise have placed constraints on the early formation of the universe.

A critical parameter is the overall density relation of the universe. If the universe possesses enough matter, gravitational attraction will eventually overcome expansion and result in a subsequent “Big crunch”. Indeed, if this were the case, it has been proposed that perhaps the universe has undergone numerous expansions and contractions prior to the current epoch. Theory suggests that this curvature should be close to zero, but observation yields a value for an open, eternally expanding universe about one third this value. Just where is the remainder of the missing universe? 

The answer comes from one of the most stunning discoveries in the past few decades. The amount of ordinary, or baryonic matter that we are familiar with is composed of protons, neutrons and electrons & turns out to be a tiny fraction of the total amount of the predicted mass-energy budget of the universe, or about 4%. 23% is composed of dark matter, and over 73% dark energy, a mysterious force that is speeding up the expansion of the universe.

Although dark energy cannot be directly seen or detected, these percentages are determined by plotting the velocity of ancient galaxies as measured by type Ia supernovae versus the current expansion via the Hubble constant seen today. This is observational proof that the expansion of the universe is indeed speeding up over time due to a mysterious form of dark energy emitting from the fabric of space itself and working over large distances. Indeed, dark energy was first purposed by Einstein, but he later rejected it as being too arcane! Dark energy does indeed appear to be a reality, and coupled with the Bing Bang Theory of cosmic evolution is currently the only model that has any predictive power in the form of observational proof. Evidence for dark non-baryonic matter is seen in motion surveys of stars and clusters of galaxies where gravity overcomes dark energy on smaller scales. Leading candidates for dark matter include MACHOs, or Massive Compact Halo Objects which could be non-luminous baryonic objects such as black holes or brown dwarf stars, or WIMPS, or Weakly Interacting Massive Particles such as the much sought after Higgs Boson. It has also been suggested that neutrinos do in fact have a tiny, as yet undetected mass, as they are much more prevalent throughout the universe and could account for the discrepancy.

The evolution of modern cosmology relies on a slow progression of our understanding of our place in the universe. New evidence derived from Type Ia supernovae and study of distant galaxies and stellar evolution has provided observational evidence for the reality of the Big Bang Theory. Further discoveries of dark energy, dark matter and cosmic inflation has led to the amazing realization that not only is most of the energy and matter in the universe composed of something that is totally beyond our everyday experience, but that the universe and time itself may continue ad infinitum, long after the last stars had burned out and the final black holes had dissipated via Hawking radiation. These revelations have brought the field of cosmology from that of conjecture to a mature science, and exploring these new and wondrous facets of our universe will be THE name of the game for years to come.

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