Hubble’s discovery is one of the greatest breakthroughs in the history of astronomy. The universe is not static. It is growing bigger and bigger. These galaxies are moving apart from one another because space itself is expanding between the galaxies. That was a marvelous discovery!
Hubble could never have made his discoveries without the hundred-inch telescope on Mount Wilson, high above the plains of New Mexico. Dark energy is one of the keys to understanding the universe’s fate. It is an invisible force pushing the universe every time and exploring a new part of the universe.
In the early 20th century, something even more incredible happened. Telescopes became powerful enough to see that many objects we thought were nebulae or other objects within the galaxy were not those things. These are entire galaxies unto themselves, like the Milky Way galaxy.
All the billions of stars in the night sky belong to one tiny island of stars out of hundreds of billions of similar and very distant little islands. We thought the entire universe was revealed to be a speck of dust in a far grander structure.
Expansion theory of the universe
The expansion theory of the universe, also known as the Big Bang theory, is the prevailing scientific explanation for the origin and evolution of the universe. Here’s a summary of the key aspects of the expansion theory:
Big Bang Event: The theory proposes that the universe originated from a highly dense and hot state, often called the Big Bang. Around 13.8 billion years ago, the entire universe was concentrated in a singularity—an extremely tiny and dense point. Then, a rapid expansion occurred, forming matter, energy, and the universe as we know it.
Expansion of Space: According to the theory, the universe continues to expand. It is not that galaxies are moving away from each other within a fixed space; rather, space itself is stretching. As the universe expands, the distances between galaxies and other celestial objects increase.
Cosmic Microwave Background: One of the key pieces of evidence supporting the expansion theory is the detection of cosmic microwave background radiation (CMB). CMB is residual radiation from the universe’s early stages, and its uniform distribution and specific temperature patterns align with the predictions of an expanding universe.
Hubble’s Law: The universe’s expansion is described by Hubble’s law, which states that galaxies move away from us (and from each other) at speeds proportional to their distances. This observation, made by Edwin Hubble in the 1920s, provides empirical evidence for the universe’s expansion.
Inflationary Period: The expansion theory also incorporates the concept of cosmic inflation. This proposes that the universe underwent an extremely rapid and exponential expansion in the earliest moments after the Big Bang. This inflationary period helps explain certain observed characteristics of the universe, such as its overall uniformity and flatness.
Formation of Structure: As the universe expands, gravity acts on matter and causes it to cluster together, leading to the formation of galaxies, stars, and other structures. The distribution and arrangement of these structures provide insights into the universe’s evolution over time.
Ever since Edwin Hubble discovered that the universe is expanding, there has been a great debate about the speed of this expansion, also called The Hubble constant. In 2000, scientists thought they had reached a consensus: a Hubble constant at about 72. But since then, two methods of calculating the expansion rate have gotten farther apart. Some scientists think it’s too big of a difference to be a statistical fluke.
In 1929, Edwin Hubble discovered galaxies aren’t stuck in one place. Not only are they moving, but they’re flying away from Earth at incredible speeds. It was the first real evidence of the Big Bang. All galaxies, on average, are moving away. It became known as Hubble’s law. His discovery is still the starting point for exploring the Big Bang. Hubble convincingly demonstrated by seeing the motion of those galaxies that the universe is expanding.
Theoretically, an expanding universe must have started from a single point. By measuring how fast the universe expands, astronomers calculated backward and figured out when it bursts into life.
Expanding universe law: A galaxy’s velocity is directly proportional to its distance.
v = Hr; Where, v = recessional velocity, H = Hubble constant, r = distance.
Hubble’s law states a correlation between the distance to a galaxy and its recessional velocity determined by the redshift.
The term “big bang” is in popular parlance. Everybody knows and talks about it, but there’s imprecision in what that means. To some people, the word “big bang” means the instant of creation. The big bang theory is the theory of the aftermath of a bang.
It’s a theory describing the early universe’s expansion, assuming it started in a very hot, dense state. It assumes that all the particles are already present and have been set into motion. The theory then describes how the universe expands and cools, how the early chemical elements formed, and how stars formed.
Evidence that the universe is expanding
We must learn about a Doppler shift phenomenon or effect to understand how expanding the universe is possible. Imagine we have some source of waves, which could be acoustic or sound and electromagnetic waves or light. These waves are emitted with a constant frequency. Now let’s say that the source of these waves is in motion to the observer’s frame of reference, like a car.
When the car approaches the observer, the sound waves will be shorter than if the car was stationary. This is because the car is moving closer to the observer in between the emission of each wave. If the wavelength is shorter, the frequency is greater, and the observer will hear the sound as pitched up.
Once the car passes and is moving away, the wavelength will increase. It is because the car will travel some distance from the observer between the emission of each wave. A longer wavelength means a lower frequency, and the observer will hear the sound being pitched down. A higher pitch coming towards the observer lower pitch moving away from the observer, and that’s why a vehicle passing by will sound variation.
Precisely the same principle works with electromagnetic waves or light. A visible light source moving toward the observer will increase its frequency, and the wavelength will decrease towards the blue end of the visible spectrum. Scientists call it the blue shift.
If it moves away from the observer, the frequency will decrease, and the wavelength will increase towards the red end of the visible spectrum. Scientists call it redshift.
These shifts are easy to observe on an emission spectrum because we know the emission lines for hydrogen and other elements. If an object gives emission lines that are shifted from these values, it’s an obvious indication of the motion of that object.
When Hubble observed the emission spectra for various galaxies, it became apparent that almost every galaxy in the observable universe is redshifted. It means they are almost all moving away from us, with the only exceptions being a handful of galaxies nearby that are gravitationally bound to the Milky Way.
Astronomers realized the universe must be expanding rather than assuming the Milky Way is an especially repulsive galaxy from which the others are fleeing. If the universe is expanding, it must have been very close together. This fact was the first evidence suggesting the universe’s birth from a single point at some specific time in the finite past.
But as if this realization wasn’t shocking enough, Hubble also realized that the recession velocities of the galaxies increased with greater distance. In other words, the farther away a galaxy is, the faster it moves away from us.
This relationship is stated in Hubble’s law, a straightforward equation relating recession velocity, measured in kilometers per second, and distance, measured in megaparsecs, equivalent to one million parsecs, or 3.26 million light-years. This equation also utilizes H, or the Hubble constant, equal to about 70 kilometers per second per megaparsec, with some dispute over its precise value.
“Once you can accept the universe as matter expanding into nothing that is something, wearing stripes with plaid comes easy.” ~ Albert Einstein
“One can imagine that God created the universe at literally any time in the past. On the other hand, if the universe is expanding, there may be physical reasons why there had to be a beginning. One could imagine that God created the universe at the instant of the big bang, or even afterwards in such a way as to make it look as though there had been a big bang, but it would be meaningless to suppose that it was created before the big bang. An expanding universe does not preclude a creator, but it does place limits on when he might have carried out his job!” ~ Stephen Hawking
“What a dull universe it would be if everything in it conformed to our expectations if it held nothing to surprise or baffle us or confound our common sense. A century ago, no one foresaw the existence of black holes, an expanding universe, oceans on Jupiter’s moons, or DNA. What could be more enriching than to know that we share a common origin with all living things, that we are kin to chimpanzees, redwoods, and mollusks? ~ Steven Pinker
“In an expanding universe, time is on the side of the outcast.” ~ Quentin Crisp
“In an expanding universe, order is not order, but merely the difference between the actual entropy exhibited and the maximum entropy possible.” ~ Kim Stanley Robinson
“There are only certain intervals of time when life of any sort is possible in an expanding universe and we can practise astronomy only during that habitable time interval in cosmic history.” ~ John D. Barrow
There’s a lot in theory, but it does not describe the bang itself. Inflation is based on ideas coming out of particle physics. It tells that forms of matter should exist at very high energies that turn gravity on its head and cause gravity to behave repulsively instead of attractively. Since the inflationary theory is the hypothesis that this repulsive gravity was the bang of the big bang. It’s what set the universe into this period of gigantic expansion.
Accelerating expansion of the universe
Astronomers found a mechanism to permit it to decay, so the universe could be smooth and end inflation. Most cosmologists would say yes, and the wonderful agreement between observations. The theory proves that the inflationary theory is right. That inflation solves cosmology problems is remarkable. But getting this self-repulsive energy to decay, end inflation, and begin the universe.
The universe’s expansion is accelerating, not slowing down as everyone expected. Cosmologists were dumbfounded. An astrophysicist Saul Perlmutter leads the supernova cosmology project. He thinks the expansion of the universe was raised. The universe was getting larger, and distances were getting greater. If it could go that way, couldn’t it go the other way? What would happen if someday it stopped getting bigger and started getting smaller? Could it collapse?
The universe wasn’t slowing at all. It was speeding up, and that was a shock. But it must have been slower in the past in expansion than it is today, and that’s an acceleration. What is the significance of this? We had not accounted for them in our physics that’s still driving the universe. That’s making it expand faster and faster. Scientists call this energy that must permeate all space its “dark energy.” Seventy percent of all the stuff in the universe is this dark energy.
The universe is homogeneous and isotropic, which continues as far as one could imagine. People would say it carried on to infinity. The idea has changed, and people believe it continues beyond where we can see, beyond the horizon. But then, at some point, there’s a giant wall.
Things are quite different beyond that, and there could be many other patches, like expanding universe with hundreds of millions of galaxies. But they would be different in some ways. There would be different physics out there. They might shine differently, have different lifetimes, and such. It goes on, for infinity is now the current set of ideas.
The multi-verse area of constant physical law is one universe in a multi-verse of, who knows, an infinite number. Einstein’s theory allows us to happen because space-time is not only curved. It can have a different connectivity structure. Some would say that one of the underlying motivations for postulating the multi-verse is dealing with the fine-tuning problem that we find in ours. One of cosmology’s most interesting well-speculative consequences is expanding the universe.
What causes the universe to expand?
The gravitational repulsion leads to an exponential expansion, meaning the universe doubles at a certain time. If you wait the same length, it doubles again and then doubles again. Getting the universe from its size at the beginning of inflation to what it needs to have at the end of inflation to include everything ultimately requires about a hundred doublings.
These doublings quickly happen using the physics of what particle physicists call grand unified theories. Then the doubling time was about 10 to the minus 37 seconds, decimal point, 36 zeros, and one at the very end.
So as inflation goes on, more and more positive energy creates a matter of various kinds. But at the same time, more negative energy is created in this gravitational field that fills the ever-expanding region of space and the balance. So the total energy remains what it was. The total energy in the stars, galaxies, and vast amounts of matter throughout the observed universe is canceled by the negative contribution of the gravitational field. So the total energy of the universe is consistent with being zero.
Now all of this matter then is created at the end of inflation. It’s created in this repulsive gravity material as inflation goes on but becomes normal. Cosmic inflation is what makes the big bang go bang. Now, digging deeper into how the expansion works? How does the nature of space affect the expansion?
Why do some scientists believe that the universe is expanding?
Space itself doesn’t respond to anything else that’s happening. It’s a separate entity. After Einstein invented his special theory of relativity, which described how space and time change according to different observers, his revised theory of gravity says that space is not inert.
Space is elastic. It can stretch, warp, wiggle, and then that opens up a possibility that it’s not fixed. But it’s changing its shape and curvature and warps with time. – Albert Einstein
If the universe expands, what will happen if it goes backward in time? So it will have contracted, and eventually, it will have contracted to a point where everything we see. All the matter and radiation in space will have contracted to a point, which means the temperature and density would have become infinite. Space would have, in some sense, disappeared, according to Einstein’s theory. It is the big bang or the cosmic singularity.
Then it led to a second problem. Imagine it has a beginning and some violent quantum event. Then what comes out of this event should be something turbulent. It should be non-uniform, hotter in one direction than another, and not look that way.
The universe looks remarkably uniform, and space seems hardly warped at all. So the moment scientists had the idea of a beginning, they had to invent a new idea to explain the smoothness that would emerge right after the beginning.
If the universe has a beginning, the only solution is a period that scientists call inflation. The inflationary theory developed historically because if there is self-repulsive energy, it could cause the universe to smooth itself. But once it began this inflation process, it didn’t have a mechanism to stop it. So the universe had this runaway inflation forever.
One person who made considerable contributions to categorizing galaxies was Edwin Hubble. He developed the categories and subcategories we use today. But that’s not all that Hubble did. He gathered data that would later interpret that the galaxies are all moving away from one another. It was determined by looking at emission spectra for many galaxies.
Overbye, Dennis (20 February 2017). “Cosmos Controversy: The Universe Is Expanding, but How Fast?”. The New York Times. Retrieved 21 February 2017.
Radford, Tim (3 June 2016). “Universe is expanding up to 9% faster than we thought, say scientists”. The Guardian. Retrieved 3 June 2016.
Slipher, V. M. (1913). “The Radial Velocity of the Andromeda Nebula.”
Vesto Slipher – American astronomer.
“On the Curvature of Space.” General Relativity and Gravitation: 1991–2000.
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