Category Archives: Universe & Cosmology

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).


 

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Decoding Light to Unlock the Secrets of the Universe

prism Much of what we know about the large-scale universe is due to decoding signals from light. The light that reaches the earth from faraway galaxies arrives as a wide spectrum, which is much more than visible light (or white light). Light is actually an electromagnetic wave with a range of wavelengths. White light is but a tiny band in the middle of the spectrum of wavelengths. When white light is refracted, such as passing through a prism, we see the colors of the rainbow.

If we move past the red band the wavelengths get longer, from infrared to microwaves to radio waves. Moving towards the opposite side of the spectrum, past the violet band, the wavelengths get shorter, from ultraviolet to x-rays to gamma rays. Even though the large majority of light is invisible to us, scientists have instruments that can detect information from the spectrum. The following list shows 5 things we know about the universe from decoding light.

1) The Contents of the Universe

Exploring the universe started simple with just visible light; ancient astronomers gazed at the night sky with the naked eye. All the twinkling yellow dots basically looked identical. They could not determine the size and distance of objects. The advent of the telescope added detail to the night sky, such as differentiating stars from planets and discovering individual galaxies.

Space telescopes, like Hubble and Kepler, are now placed in the earth’s orbit. From above the atmosphere, these and other instruments are collecting a tremendous amount of details about the universe. For example, the Hubble Space Telescope focused on a dark spot in the sky for a period of 10 days. In this tiny patch (roughly the opening of a drinking straw) an image of 10,000 galaxies was produced.

2) The Existence of Extrasolar Planets

In recent years, over 1,700 planets outside our solar system have been discovered (some of them are earth-like). Most planets orbit stars, therefore planets can be detected by examining changes in starlight, which are caused by existing planets. Astronomers use a number of methods to find planets. The two most effective methods are:

  1. The transit method: By observing a star for a period of time a planet will occasionally pass in front of the star. Viewed from earth, a planet will cause the starlight to dim slightly, thus announcing the planet’s presence.
  2. The Doppler method: As a planet orbits a star it exerts a gravitational effect on the star, which causes the star to wobble slightly. This can be detected by examining variations in the light spectrum as the star moves towards or away from the earth.

3) The Chemical Composition

Light from distant stars and galaxies can be converted into a spectrum of colors. This is achieved with an instrument called a spectroscope, which is attached to a telescope. This is perhaps the most valuable tool for decoding light. As it pertains to the chemical composition of the universe, a particular property of light contains precise information from its source.

When the light spectrum of a star is displayed by a spectroscope, vertical lines (called absorption lines) appear at specific locations, depending on the elements contained in the star. Each element produces a unique pattern of lines, which can be matched with experiments in a laboratory. Even though the information contained in the spectrum will be from a number of elements, the distinct pattern of each element can be sorted out.

Absorption lines

Absorption lines

By studying the information from light, astronomers have found that all the stars in the universe have more or less the same chemistry (including our sun). Thus, knowing that all the elements originate in stars, the chemical composition of the universe is essentially the same everywhere. The elements found here on earth are plentiful in other galaxies as well, leaving us to speculate that other life-sustaining planets may be out there.

4) The Universe is Expanding

There is another valuable piece of information from the spectroscope that has transformed our view of the universes. It is called a redshift. When the spectrum from distant galaxies is examined, the vertical lines are shifted towards the red end. This is due to the Doppler effect or the Doppler shift, and it has to do with the nature of waves. Light is a wave, and similar to sound waves, incoming waves will be stretched when the source is moving away; thus causing the absorption lines to shift towards the red end of the spectrum.

Redshift

Redshift

The conclusion from this information is that galaxies are moving away from us. The universe is expanding. The exception to this rule is that nearby galaxies are not expanding, because they are held together by gravitational forces. But for the universe as a whole, galaxies are moving away from each other. In other words, the earth’s location is not unique; the view from any location in the universe would be similar. Incidentally, it is actually the space that is expanding. The galaxies are rushing away because they are being pulled by the swelling of space.

5) The Big Bang

There are 3 ways we can get to a big bang origin of the universe by studying light:

  1. The expansion rate: The universe is expanding at a defined rate (based on the redshift), which is simply stated as: distant galaxies that are twice as far away from us are moving twice as fast, and galaxies that are 3 times as far are moving 3 times as fast. This means that if we reverse the timeline, in the distant past all the galaxies would converge at a point of infinite density. This was the moment of creation.
  2. The cosmic microwave background radiation: The CMBR is the remnant of the intense energy that was created at the big bang. The light from the big bang event has propagated throughout space, and is presently detectable as microwave radiation. Although the radiation is now faint, it is present in all directions of space.
  3. The agreement between prediction and observation: The amount of lighter elements (hydrogen, helium, deuterium and lithium) that are now present in the universe agrees with the predictions of the big bang theory. The quantity of these elements were detected from light coming from old stars and distant galaxies. The amounts are consistent with what the theory predicts would have been created in the early universe.

From an everyday perspective, light illuminates the world and we see things as a consequence. However, when we examine the large-scale universe our eyes alone are not sufficient. It is remarkable that light from very far away contains information from its source. And if not for ingenious techniques in decoding light, figuratively speaking, we would forever remain in the dark.

 

References: Richard Dawkins, The Magic of Reality

Lawrence M. Krauss, A Universe from Nothing

Stephen Hawking’s Universe -101- Seeing is Believing (June 14, 2013) https://www.youtube.com/watch?v=5kgPxvJqvEA
The Big Bang: Observational Evidence (June 4, 2012) https://www.youtube.com/watch?v=8WaI-iIlgdI


Discovering the Nature of Gravity

Of course we all know what gravity is. It’s the force responsible for making objects fall, keeping our feet firmly planting on the ground, and maintaining the moon’s orbit around the earth. But by what mechanism does gravity accomplish these tasks? Surely there are no invisible strings of a master puppeteer. The full story behind understanding the force of gravity spans at least 400 years. Three giant steps have led to modern physics’ current picture.

Heliocentric Modle

 Step 1: The Copernican Revolution, Galileo and Kepler

Just before he died, in 1543, Nicolaus Copernicus published his famous work describing the heliocentric model of the universe. Although he had formulated his theory years earlier, he delayed publishing until the end of his life. This was probably because he feared criticism from contemporaries or retribution from the church. Placing the sun at the center of the known universe (as opposed to the earth) was a revolutionary idea for its time. This was a monumental leap in the early scientific age.

The idea that the earth moved went against common sense and intuition. In reality, whether the sun moved or the earth moved could not be determined by visual means. Sometimes science has to rely on other methods; in this case, the daily/monthly movements of the planets had to be charted and analyzed.

An object can only be said to be in motion in reference to something else. For example, if you are on a boat that is departing from a large dock, and you look to your side, you will see the dock moving. For an instant you will think that the dock is moving. Then you realize this can’t be true. You may feel the boat rocking or accelerating, but from a visual point of view you can’t tell which is moving.

Years later, Galileo adamantly supported Copernicus’ view and took the brunt of the attack from the church. He was sentenced to house arrest, where he spent the last decade of his life. Nevertheless, Galileo’s contribution to science extended much further than the celestial model. He was instrumental in establishing observation and experimentation as pillars of scientific reasoning. It was becoming clearly that there was order and predictability in nature, which was accessible to human analysis.

Johannes Kepler also lived in Galileo’s time, and he was able to calculate the motion of the planets using mathematics. His most famous work is known as the laws of planetary motion, a precursor to Newton’s laws. In the process he calculated that the orbits of the planets were not perfect circles as originally thought. But rather moved in elongated circles called ellipses. Although the movement of the celestial bodies were being charted in great detail, there was still no comprehensive theory of gravity.

 Step 2: Newton’s Insights

Newton's Cannon

Newton’s Cannon

Isaac Newton imagined a cannon perched on a mountain top and asked himself the following question: what would happen if cannon balls were fired at steadily increasing speeds? The first few balls would start out in a straight line and then fall to the earth in a curved trajectory. However, if he kept going, something peculiar would happen. The curved path of the cannon ball would eventually match the curvature of the earth. The cannon ball would be in perpetual free fall, and orbiting the earth.

This was the key insight. The same force that was responsible for maintaining the orbits of the moon and planets also caused an apple to fall from a tree. No one had thought of this before. At least if someone had, it did not become public knowledge.

Therefore, the story that Newton got his idea of gravity when an apple fell on his head may not be true. He could have been thinking about cannon balls. But having a cannon ball fall on his head does not make for an inspiring story. What followed was a mathematical unity of both the heavens and earth, his laws of motion and universal gravitation. In spite of Newton’s great achievements, he still had no clue what gravity actually was. It would take more than 200 years for someone to come up with the answer.

 Step 3: Einstein’s Imagination

Among many things, Albert Einstein was famous for his thought experiments. He imagined physical scenarios, which he tried to figure out what would happen and how it could be explained. Perhaps this is how he came up with his picture of gravity.

In 1915, ten years after his theory of special relativity, he published the theory of general relativity. As it relates to the actual cause of gravity, the answer is as counter intuitive as the earth moving through space. The gravitational effects are caused by the properties of space itself; just as Einstein had shown that time was flexible (in special relativity), space was also flexible.

It is the warping or curving of the fabric of space that make objects fall and maintain the orbits of celestial bodies. It is similar to the effect of a large rubber sheet (like a trampoline). If one were to place a large heavy ball at the center of the sheet, any smaller balls would be drawn to it by the warping of the sheet (caused by the heavy ball).

Warps in Space

Warps in Space

Orbits will be created when a balance is established between the motion of a body and the distortion of the spatial fabric. That’s it, distortions in space caused by massive bodies, not a pull or push is responsible for gravity. This theory goes beyond Einstein’s imagination; it has been confirmed by scientific observations. It took 400 years of investigation to understand the basic property of one of the most familiar forces on earth.

 

References: Richard Dawkins, The Magic of Reality

The Elegant Universe 1 of 3 Einstein’s Dream (Published on Jun 21, 2012) https://www.youtube.com/watch?v=UV_X2B5OK1I

Stephen Hawking’s Universe -101- Seeing is Believing (June 14, 2013) https://www.youtube.com/watch?v=5kgPxvJqvEA