---------------- 1994 - How many galaxies are there?
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- Our Milky Way Galaxy is enormous. It is over 100,000 light years across. In our Local Group of galaxies, all are gravitationally inter-connected, there are over 20 galaxies. The biggest is not our own galaxy, it is our neighbor the Andromeda Galaxy. Under good conditions, in the northeast night sky, it is visible with naked eye.
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- All those stars you can see are all part of our Milky Way Galaxy. How many do you think you can count with the naked eye? What is your guess?
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- The answer is 2,000.
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- Now with the best telescopes we have how many galaxies can we count? The answer just increased by a factor of 10 due to the latest observations.
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- The galaxy is the basic unit for containing all the matter that is in the Universe. What we see is Ordinary Matter. But, what cosmology theory tells us is that what we observe is only 5% of what is out there in the form of the Universe’s total mass-energy. Remember, mass and energy are two different forms of the same thing. There is 20% that is in the form of Dark Matter. And, 75% is in the form of Dark Energy. That is 95% that I am not writing about because it is still a mystery. We really don’t know what it is?
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- Observations were greatly changed when the Hubble Space Telescope was used to take a very long time exposure of one point in the sky. After gathering light over many days staring at this same spot. In this small area in the sky astronomers could count 2,000 galaxies.
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- This was in 1995 and in only a few arc-seconds of the telescopes field of vision they counted these 2,000 galaxies. Multiplying this patch to the area of the entire sky the count would be 200,000,000,000 galaxies. ( Request Review about the Deep Field Observation to learn more.)
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- Over the next 15 years deeper and wider images have been obtained. The real breakthrough was not deeper and wider but use of the infrared part of the light spectrum outside the wavelengths of visible light. In 2009 astronomers began observing using many different wavelengths. Next the galaxies were categorized by their light wavelengths. This data could also be used to calculate the distance to these galaxies. With the new data they could create a 3-D map of the expanding Universe.
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- The distance can be calculated from the Doppler Shift in the light spectrum. The greater distances appear to be moving away from us faster. This technique used to calculate distances is referred to as the “photometric redshift” . The wavelength of a light source moving rapidly away from us gets lengthened , or shifted toward the red end of the light spectrum.
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- With this distance information astronomers can determine how many galaxies at a given mass are spread within each volume of space. They found out that galaxies have a regular distribution in space, ie: it is not a random distribution. High-mass galaxies have a Gaussian-law distribution. Low-mass galaxies have a Power-law distribution. Characterizing galaxy luminosities and mass over time allow astronomers to estimate how many galaxies there are using this new data.
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- The conclusion is that the further back in time you go, at higher and higher redshifts, the more low mass galaxies you find. Therefore, tiny galaxies must have merged into larger galaxies over cosmic time. For every one galaxy that exists today there were 10 galaxies back when the Universe was only 1 billion years old. Today it is 13.85 billion years old.
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- The total count of galaxies over all time is 2 trillion.
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- That is 2,000,000,000,000 galaxies each with billions of stars. That brings us to “Olber’s Paradox” Why is the sky dark? With that many stars in every line of sight the sky should be lit up. If the Universe is infinite in time and space every point in the sky would be occupied with light. So, why is it dark?
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- The reason is the Universe is expanding. Light travels at a finite speed, 186,000 miles per second. Some star light has not had enough time to reach us because it started from so far away.
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- Another reason is the Universe expansion is stretching light wavelengths into the infrared part of the light spectrum. Our eyes can not see wavelengths broader than 700 nanometers wavelength. That is the technology breakthrough, infrared detectors in telescopes, that has allowed us to see the more distance galaxies. This Doppler Shift in wavelengths is further proof the Universe is expanding. More space is continually being added. All the galaxies, not gravitationally coupled, are moving away from each other across all of space.
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- Our ability to detect fainter and younger galaxies is continuously improving. As deeper data emerges better estimates of galaxy count will be made. Counting over the history of the Universe today’s estimate is 2 trillion galaxies. This is a factor of 10 higher than we thought previously.
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- Stay tuned, there is still more to learn.
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----- 707-536-3272 ---------------- Wednesday, January 3, 2018 -----
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