--------- #1234 - Why is the Sky Dark and the Universe Leaving Us?
- Trying to describe the size of the Universe is trying to explain the unknowable. But, there is a lot to be learned in trying. The expansion of the Universe explains a lot of what we observe. The Cosmic Microwave Background was a critical discovery in helping us determine the age of the Universe and the rate of the expansion of space.
- Attachments - Big Bang expansion.
- How big is the “Whole Universe”? We do not know. How you answer the question depends on the theories you use for the expansion of the Universe. The idea here is to try to understand what we do not know. Your guess is as good as mine. But, let me try to explain what I think I know from several different directions. Let’s start with the “ Observable Universe”.
- The Universe is 13.7 billion years old. Light has been traveling for 13.7 billion years since it began. The Cosmic Microwave Background radiation is our best evidence of this and light from it began about 380,000 years after the Big Bang. I will cover this topic some more in later paragraphs. So, we can say the radius of the “Observable Universe” is 13.7 billion lightyears. This is the distance light can travel in 13.7 billion years. One light year is 5,880,000,000,000 miles, so that is 81,000,000,000,000,000,000,000 miles light has traveled in 13.7 billion years. This sounds straightforward but going from time of travel to distance assumes that the velocity is constant and the geometry of straight lines. We think the geometry of the Universe is “flat” or nearly “flat”. If the “ Whole Universe” is really big than the “Observable Universe” would appear flat, similar to the horizon on the planet Earth.
- The horizon to which we can see in the Universe is not the same as the horizon we see on Earth. The horizon in the Universe is a “time” horizon, not a “space” horizon. No matter which direction you look you see the horizon of the beginning of time and that horizon encompasses a certain volume of space.
- At some time later you see the horizon which is the beginning of time, but, it now encompasses a larger volume of space because during that time space has expanded. You can not look past the “horizon” because you can not look back to a time before the Universe began.
- It is best to visualize expanding space as carrying along the galaxies for the ride, rather than thinking of galaxies as projectiles flying through a static Universe. It is this expanding space that stretches the light’s wavelengths and causes us to see a “redshift” in the light from distant galaxies and the most distant Cosmic Microwave Background radiation. The light left the CMB as Gamma Rays with 0.00011 centimeters wavelength and arrived here as microwaves with 0.11 centimeters wavelength. The wavelengths were stretched by a factor of 1,000 during their travels to us.
- The Universe as far as we know does not have an edge. But, it does have an “horizon” beyond which we cannot see. 13.7 billion years is as far back as we can see. The Universe is this old and that is how long photons of light have been traveling. But, during the time that photons were traveling that long and that far the space behind them and ahead of them has been expanding. It is like compound interest. The more space expands the more space there is to expand even more. Photons that are 13.7 billion years old are now 47 billion lightyears away from where they started. (The latest calculations put this at 78 billion lightyears away because the expansion is believed to not just be constant, it is speeding up. This has to do with the concept of Dark Energy that is accelerating the rate of expansion of the Universe. (another topic for a different review )). So, the “Observable Universe” could be at least 94 billion light years across, or, it could be 156 billion lightyears across depending on what you use for the rate of space expansion. Even if we say the “ Observable Universe” is now 47 billion lightyears in radius, we can only see 13.7 billion lightyears of that radius. We live inside a visible bubble inside a bigger Whole Universe bubble.
- If we can see 13.7 billion lightyears in one direction and we can see 13.7 billion lightyears in the opposite direction we see the same things. The same conditions exist with two places 27 lightyears separated. There is no way light could have traveled between these two places. How can they be the same temperature, in equilibrium, if the two places have never been close enough together for even the speed of light to reach them? The answer given is that they were together at one point in time. Everything was in temperature equilibrium, then, separated great distances faster than the speed of light. This theory is called Cosmic Inflation. It explains how opposite horizons can be in equilibrium. It explains why the “ Observable Universe” is inside a bigger “ Whole Universe”. The size of the “Whole Universe” is unknown and it could be infinite in size, we don’t know, though infinity is hard to grasp.
- Cosmic Inflation occurred at 10^-32 seconds after the Big Bang when the temperature of the Universe was 10^27 degrees Kelvin. The expansion of space at that instant was much faster than the speed of light.
- The theory as to why “Cosmic Inflation” occurred has to do with the separation, or freezing out, of the four forces in nature. At the instant of the Big Bang all four forces were a single force. First, gravity separated out at 10^-44 seconds and a temperature of 10^32 Kelvin. Next, the Strong Nuclear Force separated out from the Electroweak Force at 10^-35 seconds and 10^28 Kelvin. The electromagnetic force separated out from the Weak Nuclear Force at 10^-10 seconds and 10^15 Kelvin. At 10^-32 seconds during this period that the forces were freezing out of the hot ionized plasma Cosmic Inflation occurred. Space expanded faster than the speed of light for a short period of time. Therefore space is much bigger than we can see.
- We do not know if the rate of space expansion today is a constant rate or if it changes over time. By measuring how fast distant galaxies are receding from us we calculate that the expansion rate today is 47,000 miles per hour per million lightyears. That means for every million lightyears a galaxy is away from us it is receding 47,000 miles per hour faster. If a galaxy is 1 billion lightyears distant it would be receding 47 million miles per hour, 47,000,000 mph.
- If this rate of expansion is a constant velocity changing with distance, then inverting that ratio would be distance divided by velocity which is time. Distance = rate * time. Inverting the rate of expansion and running the slope backwards will take us back to zero time, the beginning of time. The invert of the rate of expansion is the age of the Universe.
----------------- one lightyear = 5,880,000 million miles
---------------- one million lightyears = 5.88*10^18 miles
---------------- one million lightyears / 47,000 miles per hour = age
----------------- 5.88 *10^18 miles / 4.7*10^4 miles per hour = age
-----------------1.3 * 10^4 hours / 8,760 hours per year = age
----------------- 13.7 billion years = age of the Universe
- Sometime after the Cosmic Expansion slowed back down to what we see today, the first light was released from the plasma. The electromagnetic energy could not escape the hot ionized plasma until the expansion had cooled enough for protons and electrons to combine into neutral atoms. This is the Cosmic Microwave Background that occurred at 12,000,000,000,000seconds, or 380,000 years, and at a temperature of 3,000 Kelvin.
- 560,000,000,000,000,000 seconds later, or 13.7 billion years later, we see this first release of Gamma Ray light as Cosmic Microwave Background radiation. The light traveled for 13.7 billion years but all that time space was expanding and the wavelengths of this light kept being stretched out. It started out as Gamma Ray radiation at 3,000 Kelvin and we see it today as Microwave radiation at 2.73 Kelvin. The temperature has decreased by a factor of 1,000 and the Universe has expanded by a factor of 1,000. Again, it is like compound interest. The more space expands the more space there is to expand more. 27.4 billion lightyears is the diameter of the “Observable Universe”. What about the “ Whole Universe”?
- We do not know. The “Whole Universe” could be infinite in size, but, that is hard to grasp. If the number of stars and galaxies were infinite the night sky would be a dome of star light. The night sky would not be dark because every direction you looked you would see a star. However, this dark sky could still be explained if the infinite Universe was expanding because the dark sky could be from those far away stars where their light has not had time to reach us. Worse yet, if the Universe was expanding fast enough at the farthest distances galaxies would be receding faster than the speed of light. Their light would NEVER reach us. The Universe, including the “Observable Universe” would be disappearing over the “horizon“. If this accelerated expansion continues for a long time the only stars we will see at that time would be those that were gravitationally bound in our Local Group of galaxies. All the other galaxies would have disappeared over the horizon, beyond the edge of light able to reach us.
- The idea here is to try to understand what we do not know. Your guess is as good as mine.
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707-536-3272, [email protected] Thursday, December 23, 2010
