Our Universe is Expanding
Our universe is expanding. I'm talking about our physical cosmological universe. And I have to make a distinction that I'm talking about our universe and not some other universe. As many physicists now believe that our universe is just one of many universes out there in the multi-verse. Hear physicists Brian Greene talk about this on NPR radio.
Expanding means that things like galaxies in our universe is moving further and further apart from each other. Not only is our universe expanding, but the rate of expansion is increasing. Our universe has been expanding every since the Big Bang. At some time in the past, the rate of expansion has slowed, but starting since about 7 billion years ago, the rate of expansion has been increasing. There is a nice chart of this rate of expansion in section "Cosmic Acceleration" on page 139 of the book The Hidden Reality by Brian Greene. And on page 152 of Visions of the Multiverse, Dr. Steven Manly writes "the rate of expansion of the universe is growing!"
The bottom line is that the expansion of the universe is accelerating. It is expanding faster and faster as time goes by.
Physicist Brian Greene
Brian Greene is a superstring theorist and studied at Harvard and Oxford and taught at Columbia and Cornell. He is probably best known for his PBS television special on the "Elegant Universe" and from his best-selling mainstream books "The Elegant Universe" and "The Fabric of the Cosmos".
Greene writes in his latest book The Hidden Reality: Parallel Universes and the Deep Laws of the Cosmos published in 2011 that ...
"For the last 7 billion years, contrary to long-held expectations, the expansion of space has not been slowing down. It's been speeding up." [page 139]
Dark Energy Causing Universe to Expand
In the past, many speculated that the universe's expansion would be slowed by gravity pulling masses together. But now many experiments have shown that universe expansion is accelerating. So something is pushing things away from each other.
What is causing our universe to expand?
One popular theory is that "dark energy" is the repelling force that is causing the universe to expand. Our universe is composed of about 73% dark energy. This is not to be confused with "dark matter" which compose 23% of our universe. That means that only about 4% of our universe is the everyday matter such as cars, trees, sun, sky, bird that we are familiar with.
What is dark energy?
Wikipedia defines it as ...
"dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Dark energy is the most accepted theory to explain recent observations and experiments that the universe appears to be expanding at an accelerating rate."
Non zero cosmological constant
In the past, physicists believed that the "cosmological constant" was zero. The cosmological constant is a number used in the equations of the cosmos by physicists including Albert Einstein equations.
However modern experiments (including the redshift observations of a type Ia supernovae in 1998) have shown that the cosmological constant was close to zero but is not zero.
The details are beyond the scope of this article. The main point being that the fact that we have a non-zero cosmological constant is consistent with the fact that the universe is expanding.
The Big Freeze
If our universe continues to expand at this rate, objects will spread further and further apart. Some star will be so far away from us, that we can no longer see them. The night sky will have fewer stars. Eventually, there may be no more star at all. And when our sun runs out of fuel and stars are too far for us to migrate too, then life must cease to exists as there will be no more energy source around. But don't worry, this will not happen in our lifetimes.
In preface of the book Parallel Worlds, Michio Kaku writes ...
"Astronomers now realize that the universe is expanding in a run-away mode, accelerating without limit, becoming colder and colder with time. If this continues, we face the prospect of the "big freeze," when the universe is plunged into darkness and cold, and all intelligent life dies out."
The "Big Freeze" is the idea that the universe will expand forever. As it expands, its temperature will decrease. And eventually, energy will dissipate to the point where life will cease to exists.
Wikipedia says ...
"Observations suggest that the expansion of the universe will continue forever. If so, the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario is popularly called the Big Freeze."
Beyond Our Cosmic Horizon
Another consequence of an universe that is expanding faster and faster is that conceivably, space can expand faster than the speed of light. You may have heard that nothing travels through space faster than speed of light. This is true. But we are not talking about something traveling through space. We are talking about space itself expanding. And it can expand faster than the speed of light. What that means is that there are parts of space that we can never access because they are so far away that even light can never reach us even if we wait forever. So there can be regions of space that is beyond our "cosmic horizon".
Some Physics Humor
A joke that was found on the back cover of the book Absolute Zero Gravity goes like this ...
"The astrophysicist's dilemma: If the universe is really expanding, why can't you find a parking place?"
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