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---------------------- 2292 - Accelerating the Universe from Unknown Force
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- In the beginning the matter was denser and the resulting gravity more intense in gathering the mass together into stars and galaxies. At the same time gravity was decelerating the expansion rate of the Universe.
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- The decelerating Universe continued for 7,000,000,000 years, but eventually, the mass became more diluted by the expansion and gravity had less and less influence. A shift occurred at 7 billion years, half the age of the Universe, and the Universe expansion rate began accelerating.
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- It has been accelerating for the last 7,000,000,000 years. This acceleration is caused by an unknown force, we call Dark Energy. The force appears to be in the vacuum of space but we do not know what causes it. It is some form of anti-gravity, or repulsive force that is expanding space itself.
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- Astronomers discovered this phenomena by measuring the light emitted from exploding stars, called supernova. When stars burn all their fuel they explode ending their life as a star. Smaller stars, like our Sun, create a smaller explosion. They become a planetary nebula expanding out as a Red Dwarf then condensing back into a White Dwarf.
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- In about 5 billion years from now, our Sun will be 10 billion years old. It will have burned all its hydrogen and helium but will not have enough mass for gravity to burn carbon and oxygen. It will expand into a planetary nebula and become a Red Dwarf with a diameter out to the orbit of Mars ( The Earth is cooked ).
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- Gravity will eventually pull material back into a core called a White Dwarf that is 90% the diameter of the Earth, and 60% of the Sun’s original mass. Imagine how dense this is?
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- Larger stars with masses greater than 8 times the mass of our Sun end their lives differently. Because of their immense gravity they continue to condense and fuse higher level elements. Elements up to iron are fused at their core.
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- Around the core are shells of silicon, oxygen, magnesium, carbon, and helium. There is no hydrogen because it has been blown away by the stellar winds. The gravity is so immense this fusion occurs more rapidly and the larger stars have shorter lifetimes.
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- A star 8 times more massive than our Sun will only live for 100 million years. A star 25 times larger will live just 3 million years, burning 80,000 times brighter than our Sun during its short lifetime.
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- There is a size limit that separates these two types of exploding stars. The limit is 1.4 times the mass of our Sun. At 1.4 solar mass the stars gravity is just strong enough to breakdown the ability of electrons to orbit their proton nucleus. The electrons break out of their shells and the stars core becomes dense with neutrons and protons packed together inside the atomic nuclei.
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- The density reaches 4 * 10^17 kilograms per cubic meter ( water is 10^3 kg/m^3 ) When the star’s core collapses in this way the unsupported shells of fusing matter above plunges inward at 15% the speed of light. The outward flowing neutrinos and rebounding core slam into this matter causing a tremendous explosion. It becomes a supernova. Its luminosity becomes 100,000,000 times brighter.
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- When a supernova explodes at a lower limit of 1.4 solar mass it explodes with a known amount of energy and luminosity. If a light spectrum is taken of this exploding supernova the elements in its shell can be identified as a 1.4 solar mass, called a Type 1 Supernova.
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- Type 1 supernova get bright up to a maximum in 20 days, dim by a factor of 2 in the next 14 days, than slowly dim 1% per day over the next 18 months. This unique time sequence of decay starts with radioactive nickel that has a half-life of 6.1 days, decaying to cobalt. Then cobalt decays into stable iron with a half-life of 77.1 days.
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- If astronomers know the brightness of a star they can calculate the distance the star is away from us. The dimmer the star the further away it must be.
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- A big potential error in measuring brightness of distant objects is the effect of interstellar dust dimming the before it reaches us. This error can be identified because interstellar dust is very fine sub-microscopic haze made of carbon and silicon. The size of the dust is such that it absorbs or scatters ( little antennas ) blue light m ore so than red light. ( Why the sky is blue and the sunset is red ).
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- If we take a spectrum of light from a supernova and blue light and red light have the same intensity and the same decaying light spectrum, then, dust absorption can be ruled out as a possible factor for dimness.
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- If we start with a Type 1 Supernova explosion in a nearby galaxy that we already know its distance from other means, then Type 1 Supernova at more distant galaxies will appear dimmer by the inverse square of the distance. A supernova three times the distance away will be nine times dimmer.
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- As the distance to a light source increases you receive less light. The amount less is equal to one divided by the square of the distance. This inverse square law is very easy to understand. Visualize light emitting from a point source into a sphere. One meter from the light shines on an rectangular area one meter on a side ( 1 square meter ). Two meters from the light the area expands to two meters on a side ( 4 square meters ). Three meters from the light the area expands to 3 meters on a side ( 9 square meters ). And, so on. As the distance increases the same light is spread over a greater area equal to the square of the distance and the light becomes dimmer by that amount.
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- The Universe today is expanding at 70 kilometers per second per mega- parsec. A megaparsec is 3,262,000 lightyears distance. If we measure the redshift of the light coming from the Supernova we can calculate the distance of the Supernova assuming this constant velocity increase of 70 kilometers per second. However, if we measure the distance of the supernova using its luminosity we find that the supernova is actually 25% dimmer than expected making the supernova more distant than expected. Because they are dimmer they must be further away. Therefore, the Universe is not expanding at a constant 70 km/sec/mps, it is accelerating even faster. This same measured acceleration occurs up to 5 billion light years away.
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- As a galaxy is moving away from us its wavelength of light becomes longer, toward the red end of the spectrum.
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--------------------------- Redshift = Change in wavelength / wavelength emitted
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--------------------------- Redshift = z
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-------------------------- Velocity of Galaxy / Velocity of Light = z^2 +2z / z^2 + 2z + 2
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- For a redshift of 1 the receding velocity of the galaxy is 60% the speed of light. This is not “common sense velocity“. It rather measures the expansion of space that has taken place while the light from the galaxy is on its way to us.
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- When you see light in a telescope, the further distance you look, the further back in time you see. Back 5 billion years we see us accelerating. If we go back 7 billion years it changes. When we observe a supernova that is more than 7 billion light years distant it appears 25% brighter than expected. It appears brighter because the Universe was decelerating its expansion at that point in our cosmic history and the supernova is closer than expected.
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- The coasting point that marks the transition from a decelerating to an accelerating universe occurs at a redshift of 0.7. Using the formula above this corresponds to a receding velocity of 49% the speed of light. 49% of 299,800 kilometers/sec is 145,601 km/sec that the galaxy is traveling away from us.
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- If the velocity/distance ratio is a constant 70 km/sec/mega parsec then dividing gives the distance of 2081 mega parsecs. Each mega parsec is 3.262 million lightyears. So the distance works out to be 6.787 billion lightyears, or 7 billion years, half the age of the Universe.
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- Many supernova have been measured this way. Supernovae erupt every 100 years in a galaxy the size of the Milky Way. The light from these supernovae are likely 20,000 light years away in the Milky Way. So, the light from 200 supernova are on its way to us now.
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- The last one seen with the naked eye occurred in 1987, not in our galaxy, but in our southern neighbor galaxy, the Large Magellanic Cloud. If galaxies were the size of dinner plates the Large Magellanic Cloud would be on the same dinner table and the observable universe would be 20 miles in all directions. If we knew which way to look we should see a supernova explosion every day.
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- The best measurements to date put the age of the Universe 13.4 + - 0.2 billion years with 27% matter and 73% Dark Energy making up its composition.
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- Today’s expansion rate, called the Hubble Constant, is 70 + - 7 kilometers/second/mega parsec. The coasting point that marks the transition from an decelerating universe to an accelerating universe occurs at a redshift of 0.7, 7 billion lightyears distant, and half way back to the Big Bang.
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- This exploration opens up many interesting topics and further questions. If you have a good question, let me know. Stay tuned, there is a lot more learn.
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- March 4, 2019 54
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-------------------------- Monday, March 4, 2019 --------------------------
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