Tag Archives: Albert Einstein

The Universe Revealed Through Modern Science

Physical laws have existed since the beginning of time, but they had to be discovered for science to become relevant. Scientific knowledge was built mainly by a series of small advances and adjustments, however, a few major discoveries by a few scientists have altered the course of the scientific endeavor. The age of modern science was pioneered by men like Copernicus, Galileo and Kepler. They began to examine the patterns in nature, and discovered that in some situations the workings of nature could be explained, and even predicted. They found that nature’s harmony was governed by physical laws, which were at least partly accessible to human comprehension. They studied the motion of objects on earth, and then turned their attention to the heavens. They charted the movement of the celestial bodies in great detail, and discovered that the motion of the celestial bodies could also be predicted. The gateway to scientific discovery had been opened—the universe would soon begin to reveal its most profound secrets.

In the early years, it was Isaac Newton’s insight that stood above all others. He discovered gravity as the force responsible for the motion of the moon and the planets. And as the story goes, the same force responsible for an apple falling from a tree. In 1687, he published the Principia Mathematica, where he disclosed his law of universal gravitation and the three laws of motion. It was a major breakthrough in advancing the scientific cause. Newton’s laws provided the foundation for what has become known as classical physics. For more than 300 years his equations have stood the test of time. In fact, Newton’s equations were all that was needed to plot the course that placed men on the moon. Although his equations provided an accurate mathematical framework (actually a very close approximation that was later revised by Einstein), Newton had no idea what mechanism was responsible for the effects of gravity. It is also believed that he regarded space, the arena of motion, to be absolute and unchangeable. He viewed time in much the same way.

It was not until the early 1900s when the mysteries of space and time, as well as the underlying causes of gravity, were addressed. Albert Einstein changed the course of history when he published his theories of special relativity (in 1905) and general relativity (in 1915). Einstein formulated that space and time are not absolutes, but have dynamic qualities associated with mass and motion. In fact, he described space and time as a unified whole, which later became known as space-time.

With special relativity, Einstein showed that measurements of time (and even distance) could differ for two observers, based on their relative motion. Time will elapse slower for someone in motion than it does for someone at rest. And the discrepancy in elapsed time will increase as the difference in the speed increases. In a sense, observers carry their own clock with them. This realization signifies another important point—that the observers would also disagree on what constitutes a given moment in time. One person’s now would be different from the other person’s now, yet both perspectives would be equally valid. Keep in mind, that it’s only when dealing with speeds approaching the speed of light or extreme distances that disagreements in time become significant. The effects of special relativity are not visibly apparent in the temperate conditions that exist here on earth; however, the earth is somewhat of an anomaly in comparison to the universe as a whole. With the universe, where extreme distances and speeds are commonplace, special relativity becomes important.

With general relativity, he showed that the effects of gravity are caused by the warping or curving of space (or space-time, but for simplicity I will use the term space). Heavy objects like planets and stars warp the fabric of space, thus creating the effects of gravity. It is similar to placing a heavy ball in the center of a trampoline. Any smaller balls placed on the surface will be drawn to the center, due to the surface being warped by the heavier ball. Bear in mind that a trampoline is a two dimensional representation of what is actually a three dimensional spatial fabric. It does, however, give us a clear visual analogy of how curved space participates in the motion of celestial bodies. In the case of planets and stars, orbits will develop when a stable balance is achieved. The earth can be thought of as moving in a straight line along a curved surface of space. Or as taking the path of least resistance along the distorted spatial fabric, which is created by the sun’s presence.

Another consequence of general relativity is that just as gravity curves space, it also curves time. But what does curved time mean? Similar to special relativity, where motion alters time, general relativity claims that gravity also alters time. When gravity exerts its influence time slows down. For instance, time passes a little slower on the surface of the earth than it does for objects high above the earth. A practical example of this effect is in the technology behind global positioning systems (GPS). The satellites that guide GPS devices have to account for both special and general relativity (general relativity producing the largest effect). The internal clocks of the satellites account for the fact that clocks on the earth’s surface run slower. If not for these adjustments, GPS devices would quickly become inaccurate; the coordinates on the ground would drift off by several kilometers each day.

Einstein’s relativity goes against our common sense perceptions, but apparently this is the reality of the universe. Einstein’s insights led to modern cosmology (the study of the origin and evolution of the universe), and our current view of the universe. Both classical physics (Newton’s view) and relativity (Einstein’s view) provide a deterministic framework. That is, if the present conditions are known, the past and future conditions can also be determined. That’s assuming that you have all the present data and the mathematical ability to do the calculations.

The next scientific breakthrough would be of a very different nature. In the mid-1930s a group of scientists were unlocking the secrets of the atom. In so doing, it led to the development of quantum mechanics. They found that the atomic and subatomic realms behave in ways that are very different from the world experienced at the larger scales. A whole new set of laws had to be developed to deal with the bizarre nature of the atom—laws that are partly governed by randomness and probabilities. Physicist Brian Greene describes the nature of quantum mechanics. He writes in The Fabric of the Cosmos:

 “…according to the quantum laws, even if you make the most perfect measurements possible of how things are today, the best you can ever hope to do is predict the probability that things will be one way or another at some chosen time in the future, or that things were one way or another at some chosen time in the past.”

The probabilities that are used in quantum mechanics are more fundamental than the probabilities that are assigned to everyday events. When we assign a probability to a game of dice or blackjack, it is based on our inability to calculate the precise conditions that will determine the outcome of the event—specifically, each roll of the dice or flip of the card. With quantum mechanics, however, even if we know all the present information possible, we still can not predict a future outcome with absolute certainty. Quantum physics describes a reality that is fundamentally uncertain, in which objects have no definite position, take no definite path, and even have no definite past or future.

Some experiments (known as the double-slit experiment and variations of it) have actually shown that a single particle, such as a light photon, can behave as if it simultaneously takes a number of different paths from a source to a target. It is debatable whether this really happens; nonetheless, outcomes are determined by the number of possible paths of the photon, whether or not they are all realized. The photon takes a definite position only when it is observed or measured (when it strikes the target). In between the source and the target, it can be thought of existing as a haze of possibilities.

This is partially explained by the idea that subatomic objects, like photons and electrons, exhibit both wave-like and particle-like properties. At times, a photon or electron can be described as occupying a wide region in space, and at other times described as occupying a single point in space. Depending on the variation of the double-slit experiment, a photon can sometimes behave like a wave and sometimes behave like a particle. Although it is not entirely clear how these results should be interpreted, physicists agree that our conventional sense of reality does not apply at the quantum level—even to a larger degree than Einstein’s relativity.

I know this all sounds absurd. Nevertheless, the predictions of quantum mechanics have produced results that are extraordinarily accurate. Quantum mechanical predictions are accurate in the sense that if a sufficient number of identical experiments are carried out, the totality of the outcomes will reflect the assigned probabilities. Yet each single experiment will generate a random and unpredictable outcome. Therefore, even with the most precise calculations possible, there is an unavoidable degree of uncertainty in quantum mechanics.

It has been said that nobody understands quantum mechanics, that even scientists that work with quantum mechanics don’t understand it. So if it’s not sinking in, don’t lose any sleep over it. In summing up: the renowned physicist Richard Feynman once wrote in The Strange Theory of Light and Matter “[Quantum mechanics] describes nature as absurd from the point of view of common sense. And it fully agrees with experiment.”

Once again our common sense is challenged by the laws of physics. From classical physics to the updating of relativity, and the weirdness of quantum mechanics, reality is proving to be difficult to grasp, as these theories give us very different views of reality. For this reason, there is a consensus among some physicists that there exists a deeper level of reality to the universe that remains undiscovered. They propose that there should be one theoretical framework that describes the universe, and not a fragmented view based on several partial theories. Einstein called this hypothetical theory a unified theory (also called the theory of everything). The quest for a unified theory became one of Einstein’s passions during his later years; however, it was not realized during his lifetime.

Today, physicists are still seeking the elusive unified theory. Our present understanding of the universe is based on the two major breakthroughs of the 20th century. 1) General relativity, which describes the large scale structures of the universe, like stars and galaxies. 2) Quantum mechanics, which describes the small scale structures, like molecules and atoms. These two theories have been very successful in their own right, but in some extreme situations they cannot be applied successfully. In some situations where large densities are compressed into a tiny region of space, an understanding of both the large and the small is required. But when general relativity is applied together with quantum mechanics, the theories fall apart. This becomes a major obstacle when trying to understand conditions such as the center of black holes and the origin of the universe where these conditions need to be considered. The big bang theory describes the events a fraction of a second after the beginning, but says nothing about the beginning or before. Without a unified theory, or a new theory altogether that can deal with this situation the cause for the origin of the universe will remain a mystery.

As we have seen, each new discovery has added a piece to the puzzle and our understanding of the universe has increased dramatically over the years. The ultimate goal of science can be nothing other than a complete understanding of the laws of nature, though it may be that mystery will forever be a part of the picture. In his 1988 book, A Brief History of Time, Stephen Hawking weighs in on the subject:

“But can there really be such a unified theory? Or are we perhaps just chasing a mirage?

There seems to be three possibilities:

1) There really is a complete unified theory, which we will someday discover if we are smart enough.

2) There is no ultimate theory of the universe, just an infinite sequence of theories that describe the universe more and more accurately.

3) There is no theory of the universe; events cannot be predicted beyond a certain extent but occur in a random and arbitrary manner.”

There may very well be limits to what humans are able to understand, but this should not limit our quest for knowledge. Where would we be today if some people hadn’t questioned conventional thinking and opened the door to greater discovery? It is due to the few who dared to challenge the beliefs of their time that many benefited. Not only in science, but in other domains as well, it is the quest for knowledge that paves the way for progress. This is the case for our lives, as well as humanity as a whole. No one knows how far we can go, and only time will tell. On this note, we can at least rest assured that the modern age of science has brought humanity out of the darkness of ignorance, and into the light of knowledge.

References: Brian R. Greene, The Fabric of the Cosmos (New York: Alfred A. Knopf, 2004), 10-11.

Richard Feynman, QED: The Strange Theory of Light and Matter (Princeton: Princeton University Press, 1988).

Stephen W. Hawking, A Brief History of Time (New York: Bantam Books, 1988), 165-166.


 

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Horizons Out of Reach

Imagine you are walking on a terrain of rolling hills; in the distance you can see the horizon. Beyond that point you don’t know what you’ll find. When you arrive at the crest of a hill a whole new landscape appears with its own horizon. This is a common metaphor used to show how knowledge is usually acquired. Each horizon reached often presents another horizon (or question) in the distance.

The story of science is one of impressive discoveries. Many horizons have been reached, but many more are yet to be encountered. No one knows how far we can go and what we will find. I hesitate to limit what might be possible, because science has surprised us time and time again. If the human race survives long enough, is there anything we can’t find out? I would think that there are some questions we will not be able to answer, but which ones? One should think long and hard before ruling anything out, which I have done. For what it’s worth, I am left with two questions which appear out of reach. I’ll get back to this later but first a little context.

Horizons Reached

At present the knowledge base is immense, but it had to be acquired. Imagine going back 100, 500 or 1,000 years and contemplating the future. It’s possible that some future discoveries could have been predicted. However, there are other findings that few saw coming. It is practically impossible to provide a full account of impressive scientific discoveries. However, there are some that immediately stand out. What follows has been mentioned in prior blogs of mine; think of it as a short list of scientific highlights:

  1. The Idea of Natural laws: At around 500 BC the ancient Greeks documented the concept of natural laws. They suggested that patterns in nature could be recognized and attributed to natural laws. This was a major breakthrough in scientific thought.
  2. The Copernican Revolution: In 1543 Nicolaus Copernicus published his theory of the heliocentric model of the universe. He removed the Earth from the center of the known universe and replaced it with the Sun. This was a significant reality check, which would influence human philosophy for years to come.
  3.   Newton’s Laws: In 1687 Isaac Newton disclosed his law of universal gravitation and his three laws of motion. Newton laid the foundation for what later became known as classical physics. Now over 300 years later, Newton’s equations still apply (except for extreme circumstances).
  4. Einstein’s Relativity: With special relativity (in 1905) and general relativity (in 1915), Albert Einstein filed in the gaps in Newton’s laws. Einstein accounted for those extreme circumstances. His contribution led to a greater understanding of the large-scale universe.
  5. Darwin’s Theory of Evolution: Charles Darwin provided an explanation for how all life evolves with his famous publication in 1859. This one basically speaks for itself; few if any discovery is more impressive.
  6. Revealing the Atomic and Subatomic Realm: Beginning in the early 1900s, several people worked on theories such as quantum mechanics and the standard model of partial physics. A realm previously inaccessible was shown to be real and would unwittingly have a significant impact on human affairs.
  7. The Big Bang: In the 1931 George Lemaitre suggested that the universe began in a single geometric point. He arrived at this by applying general relativity to the observations of William Hubble. Lemaitre`s idea would eventually provide us with a truly universal origin story. 
  8. DNA: In 1962 James Watson, Francis Crick and Maurice Wilkins won a Nobel Prize in medicine for the discovery of the structure of DNA. This opened up a whole new science, which will undoubtedly impact us for generations.

Of course the list above could be significantly longer and still fall short. However, I present it just to give you a feel for how knowledge, particularly scientific knowledge, alters our perception of the world. It is debatable how many past discoveries could have been foreseen; nonetheless one can imagine some horizons in the distance which may be attainable. For example: figuring out how life on Earth got started, or the discovery of life elsewhere in the universe. Closer to home, perhaps finding a cure for cancer (or most cancers), and maybe even weather forecasting weeks or months in advance. No one knows for sure which findings are coming, but I feel fairly certain that at least two questions will remain unanswered.

Contemplating the Unanswerable

The two questions I am referring to are as follows: 1) Why is there a universe in the first place?  2) Why is the universe the way it is and not some other way? Another question which I feel I must address before moving on to question 1, is this: Why is there something rather than nothing? You’ve probably heard this one before, and it is similar to question 1. However, I find this to be a peculiar question and here’s why. First let’s define what is meant by nothing. If by nothing, one assumes the absence of everything, then nothing is a non-entity. In other words, how can nothing be a reality if by definition nothing has no existence. The question gives us two options, something or nothing and it seems to me that something is real and nothing is not. By this logic one could conclude that there has to be something, but why a universe?

For some the existence of the universe doesn’t seem to be a big problem to solve. The standard answer is that God created the universe and that’s it. However, I can’t help but ask two simple follow-up questions: a) Why is there a God in the first place? b) Why is God the way he (she, it) is and not some other way? Do you see how this works, by inserting God as the explanation for the universe we’ve circled back to where we started. In essence the questions are identical. We have merely moved the starting point from the universe to God.

Another approach is to examine the possibility of a multiverse. There are scientific reasons that suggest that other universes may exist, but that is as far as it goes.  Although the multiverse is theoretical, it may shed light on question 2. Why is the universe the way it is and not some other way? If multiple universes actually exist, it could be that all possible universes exist, therefore it is not surprising that at least one universe is like ours. Although the multiverse idea is somewhat satisfying on the surface, it has its problems. For starters, it does not address question 1. Why is there a universe in the first place? It says nothing on why there would be a multiverse in the first place.

There is also the problem of testing the multiverse idea scientifically. How can we ever verify something outside the boundaries of our vast universe? Hypothetically, even if our science advanced to a point where universes outside our own could be detected, how could we know the full-scale of a multiverse? We would likely be unable to determine how many universes exist in total. Ultimately that’s where I think the multiverse idea falls short in terms of answering question 2. Why is the universe the way it is and not some other way? If we can’t know how many universes exist in total, we can’t explain why our universe is the way it is and not some other way. All possible universes have to exist in order for the multiverse to the job. Or at the very least, it would take an extremely high number of universes.

Why is there a universe in the first place and why is the universe the way it is and not some other way? I have thought about these two questions philosophically, religiously and scientifically and have made little progress. Each approach gains momentum only to fall short. There are undoubtedly still many horizons within our reach and it will be interesting to see what lies ahead. That being said, I have to conclude that there are at least two horizons that seem to be hopelessly out of reach.

References:http://www.bbc.co.uk/schools/gcsebitesize/science/add_edexcel/cells/dnarev3.shtml

https://www.quora.com/What-is-the-relationship-between-the-Standard-model-and-Quantum-field-theory


 

The Paradox of Wave-Particle Duality

blue light beam The wave-particle duality of light and other subatomic particles, such as electrons, is a central concept in quantum mechanics. The idea that light and elementary particles have both wave-like and particle-like properties is just one of a number of strange quantum realities. The quantum revolution, which began at the turn of the 20th century, has transformed our world from a technological standpoint. A century later the quantum laws underpin our modern technology. But scientists that were probing the atom in the early 1900s were simply trying to understand the nature of reality at the smallest scales. The challenges proved to be immense, mind-boggling and paradoxical. Wave-particle duality is one paradox that is still not completely understood.

The Photoelectric Effect

Albert Einstein is most famously known for the theories of special relativity, general relativity and the equation E=MC². However, in 1905 he won the Nobel Prize for his explanation of the photoelectric effect. Before Einstein, light was generally thought to behave like a wave, similar to a water wave. But there were some unsolved questions regarding properties of different colored light. Specifically, the ability of ultra-violet light to remove an electric charge from a metal plate (a phenomenon not observed with red light).

Photoelectric Effect

Photoelectric Effect

Einstein proposed that light was composed of packets of energy called quanta (later known as photon). These particles of light acted like miniature billiard balls, knocking the electrons off the metal plate. According to Einstein, the particles from the red light carried low energy, because red light has a low-frequency. Conversely, the higher frequency ultra-violet light contains higher energy particles, which were able to dislodge the electrons from the metal plate. With the analogy of the billiard balls, it was like the ultra-violet particles were heavier than the red light particles. Therefore, the heavy particles of light were able to knock off the electrons, while the lite particles could not.

Einstein’s explanation of the photoelectric effect showed that light was made up of individual particles. It opened the doorway to a new branch of physics. Although Einstein played a key role in the foundation of quantum physics, he never accepted the implications that the theory would eventually bear out. The idea that the quantum world was ruled by uncertainty, did not sit well with him. Einstein supported the classical view of physics, where precise predictions and conclusions could be made. Referring to the probabilistic foundation of quantum mechanics, Einstein said: “God does not play dice.”

 The Double-Slit Experiment

The discovery of the wave-particle duality of light was only the beginning of the paradoxes that would later emerge. A simple experiment, known as the double-slit experiment would overthrow any common sense notion of the quantum realm. The experiment worked as follows: An electron gun was set up to fire an electron beam through a barrier with two open slits. A full screen was placed a small distance behind the barrier. One would expect that the electrons that go through the slits would strike the background screen and produce two bands. However, the outcome showed not two, but a number of bands across the length of the screen; a striped pattern emerged.

Double-Slit Experiment

Double-slit Experiment

The electrons were behaving like a wave; the stripes were consistent with an interference pattern. This had already been observed in water waves. For instance, when two ripples in a pond meet they interfere with each other, causing the similar interference pattern that was observed with the electrons. Water is composed of individual molecules and together they combine to form a wave. Similarly, the electrons were exhibiting both wave-like and particle-like properties (this was also observed in light).

If this was not strange enough, the next step of the experiment would reveal a greater paradox. When the electron gun was allowed to fire one electron at a time, the screen in the back would eventually show the same striped pattern. How could single electrons produce an interference pattern? To point out how strange that was: it was like a single electron was passing through both slits at the same time, or like each electron was carrying information from the wave as it was passing through the slits.

Explaining the Impossible

Niels BohrDanish physicist Niels Bohr, one of the founders of quantum mechanics provided a possible explanation. It is known as the Copenhagen Interpretation. According to Bohr, the electrons that travel from the gun to the screen cannot be viewed as single point particles, but rather as a probability wave. In other words, an electron exists only as a spectrum of possibilities when it travels. It carries with it every possible path from the gun to the screen, including passing through the two slits at the same time. Only when it strikes the screen is the electron forced to take an exact position.

In this view, each electron will strike the screen at a different point, however, with a sufficient number of electrons the striped pattern will emerge on the screen. Whether the electrons are traveling in a continuous beam or as single travelers, the outcome will produce an interference pattern. Even if it is not completely understood, wave-particle duality is a fundamental property of the quantum world.

It is safe to say that Einstein and Bohr disagreed as to what is ultimately responsible for quantum uncertainty. For Bohr it was enough to apply a workable mathematical framework (based on probabilities), but for Einstein there must have been an undiscovered classical principle guiding the process. A century after Einstein and Bohr there is still no classical physical principle (one that agrees with common sense) that explains the uncertainty of quantum mechanics. However unsatisfying, it seems that Bohr’s explanation of the double-slit experiment is still as good as we have. Nevertheless, Bohr realized the gap between quantum mechanics and everyday experience, he said: “If quantum mechanics hasn’t profoundly shocked you, you haven’t understood it yet.”

In time, Bohr’s approach would lead to a revolution in technology. Even if exact outcomes cannot be known, physicists can calculate probabilities that will allow electronic devises to work. Similar to the double-slit experiment, even if the path of each individual electron cannot be known, the overall pattern can be predicted. Today, computers, mobile phones and GPS devices operate based on quantum mechanics.

 

References: The Secrets of Quantum Physics Episode 1 Einstein’s Nightmare BBC Documentary 2014. Published on Feb 28, 2015. https://www.youtube.com/watch?v=uV8oSgMhS54

Brainy Quote, 2001-2015. http://www.brainyquote.com/quotes/authors/n/niels_bohr.html

Brian Greene, The Fabric of the Cosmos.


The Cosmological Constant: From Einstein to Dark Energy

EinsteinThe cosmological constant has its humble beginnings with Albert Einstein’s theory of gravity. In 1915, after a decade of working on some unsolved issues regarding gravity, Einstein completed the theory of general relativity. Today, this is still the best theory we have for describing how gravity works at large scales. Nevertheless, 2 years later (in 1917) Einstein made a small adjustment to the equations of general relativity. He introduced a term called the cosmological constant, which represented a repulsive force to counteract the attractive force of gravity.

Einstein realized that general relativity would require the universe to either be expanding or contracting, however, the belief at the time was that the universe was essentially static and eternal. Because gravity causes large structures to attract each other, logic would deduce that the universe as a whole should be contracting. But this was neither observed nor part of conventional thinking. The cosmological constant, a repulsive force with just the right value, allowed the universe to remain static. Although the cosmological constant was present in all of space, Einstein provided little details concerning what this mysterious force actually was.

Einstein’s Greatest Blunder

In 1929, Edwin Hubble carefully studied light from distant galaxies. He calculated the distance of the galaxies by examining the luminosity of a specific type of star, known as a Cepheid variable. The light from a Cepheid displayed a distinct pulsating pattern, which could be used as a distant indicator.

Hubble expanded on the work of astronomer Vesto Slipher, who was the first to observe the redshift of distant galaxies (although they were called spiral nebula at the time, because it was not yet known that other galaxies existed beyond our Milky Way). The redshift meant that incoming light waves were stretched, indicating that the observed light was moving away. This provided evidence that the galaxies were moving away from the earth. And even more significant, Hubble found that all galaxies were also moving away from each other.

Hubble’s observations confirmed that the universe was expanding. Upon learning the news, Einstein went back to his equations and removed the cosmological constant, as it was no longer needed to maintain the former belief of a static universe. It has been reported that Einstein called the cosmological constant his “greatest blunder.” Despite Einstein’s claim, the cosmological constant would resurface many decades later, but it came as an unexpected turn of events.

The Universe is Accelerating

galaxyAs of 1998 the expansion rate of the universe over cosmic time was still unknown. Either the universe would continue to expand forever, or the gravitational effects of galaxies would cause the expansion to slow down and perhaps stop. If at some time the expansion did stop, then it would stand to reason that gravity would cause the universe to collapse. This would lead to something like the opposite of a big bang (a big crunch).

The rate of expansion will determine the future fate of the universe. But how can one determine the expansion rates at different time periods? How can we know how the current expansion rate compares with past rates? Fortunately, the universe is extremely large and extremely old. Light from faraway galaxies can take millions and billions of years to reach the earth. This allows astronomers to go back in time and examine galaxies as they were in the past. The light we see now was emitted many years ago; these stars and galaxies appear as they once were.

Two international teams, one lead by Saul Perlmutter, the other by Brian Schmidt, set out to determine the expansion rate over cosmic time. They applied some creative methods based on a specific type of exploding star, called a Type Ia supernova. At the end of their lives these particular stars explode in a consistent pattern, which signal an intrinsic brightness. The astronomers determined a star’s distance from earth using the information from a Type Ia supernova. Then they calculated the redshift of the star’s host galaxy, and made the calculations with a number of galaxies at various times in the past.

Accelerating universeThe two teams eventually arrived at the same conclusion. The galaxies are currently receding faster than they were in the distant past; the universe is accelerating! This was an unexpected result, as it was mostly assumed that the expansion was slowing down over time (due to the attractive force of gravity).

The Return of the Cosmological Constant

If gravity is an attractive force, then what could be causing the universe to speed up. Enter the cosmological constant or its reincarnation, dark energy. Einstein’s hypothesis of a repulsive force that was counteracting gravity may not have been far off base (though his reason for introducing it was misguided). An unknown form of energy in empty space seems to be responsible for the acceleration of the universe. It has been dubbed dark energy because it does not emit light, but it could also be a term that points to the mysterious nature of this type of energy. Dark energy does, however, make up 70% of the total energy of the universe. Remarkably, this has been calculated and it seems to describe the universe we live in.

One more point of note: Since dark energy/cosmological constant is presumed to occupy all of space, its overall influence increases as space expands. Therefore in the distant past, when the universe was more condensed (relatively speaking) attractive gravity was dominant. The expansion of the universe slowed down at some point. However, as space swelled and galaxies moved farther apart, the dark energy caught up and then surpassed gravity as the dominant force. The tables turned, causing the universe to speed up.

Current evidence supports a cosmic story in which the universe will continue to expand practically forever. Galaxy clusters, like our local group, will still be held together by normal gravity, because they contain enough matter. However, in the far future all evidence from beyond our local group will disappear. The universe will be comprised of a bunch of island universes.

 

References: Mysteries of a Dark Universe: Uploaded on Oct. 31, 2011. https://www.youtube.com/watch?v=QUpWCRadIIA

Brian R. Greene, The Fabric of the Cosmos (New York: Alfred A. Knopf, 2004).