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--------------- 1852 - How do we measure Cosmic Inflation?
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- The Universe keeps getting bigger. Let’s say you graduate from college and you decide to pursue your dream. You dream is to answer 3 simple questions: How big is the Universe today? How fast is it growing? How old is it?
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- How fast is it growing has been given a name. It is called the “Hubble Constant” It is named after Edwin Hubble who first discovered in 1929 that there were galaxies outside our own galaxy and the distant ones are all moving away from us. The farther away the faster they were receding. And, this ratio was a “ constant”. We call it a constant because it appears to be a constant today. We don’t know if it was the same in the past or will be the same in the future. In fact it appears to be accelerating, not a constant velocity?
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- Edwin Hubble died in 1953 but the space telescope named after him is still trying to answer these questions. Today, it is measured as for every 3.26 million lightyears distance the galaxies are receding with a velocity of 74 kilometers per second faster.
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- The Hubble Constant is usually written as 74 kilometers per second per mega parsec. The mega parsec is 3.26 lightyears distance. Translating this to highway speeds we get 47,000 miles per hour for every million lightyears distance. Astronomer’s today believe this number is accurate to within 4.3%
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- So that is how fast we are moving. Not sure what direction, but, expanding “outward”. Expanding into what I am not sure? But, if space is expanding everywhere at the same time maybe there is not a “direction” per se. Whatever “outward” means. Since all of space is expanding in all directions at once, it is like a tumbling astronaut in space trying to decide which direction is “up”?
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- Ok, how about how old is the Universe? The Universe makes sense to today’s astronomers if it is 13.7 billion years old, 95% full of Dark Matter and Dark Energy and speckled with galaxies that grew from gravity from random microscopic fluctuations in the Quantum Soup created in the Big Bang. ( The microscopic fluctuations are called the Uncertainty Principle). We are here in the remaining 5% trying to comprehend the rest of this stuff.
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- How do astronomers make these cosmic measurements and calculations?
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- Looking into the dark night sky we might assume the bright stars are closest and the distant stars are faintest and the farthest.. Apparent brightness (b) can be a simple calculation for distance (r). First we need to know the Absolute Brightness (AB) of the light source. Then:
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---------------------------------- b = AB / r^2
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--------------------------------- brightness falls off as the square of the distance.
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- But, how do you know the Absolute Brightness of all these stars? Some big, some small, some are binary stars, some are single stars? Well, we only use a certain type of star, called a “ Cepheid Variable “ star. This particular star dims and fades in a particular saw tooth pattern. The more luminous (Absolute Brightness) the longer the cycle of this pattern. So, a winking star is broadcasting its luminosity and we can calculate the distance to that particular galaxy.
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- But, at very great distance even Cepheid Variable stars are too dim to discover. We need something brighter to be the “standard candle“. The new choice is the Type 1A supernovae explosions that occur with a standard luminosity.
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- Type 1A supernovae are brilliant enough to be seen across the Universe. They occur when a binary star, two stars orbiting each other, one a White Dwarf star stealing mass from the other star. When the White Dwarf reaches a mass of 1.4 Solar Mass ( 40% larger than our Sun), the gravity at 1.4 Solar Mass breaks down the electromagnetic force holding electrons and protons apart collapsing the two into neutrons at the nucleus. This causes a collapse of the entire star where all the outer mass slams into the core. This in turn causes a bounce, an enormous explosion that should have a know mass and a known brightness every time it happens. (See footnote 1).
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- Now we have Cepheid Variable stars and Type 1A supernovae as standard candles to climb the distance ladder across the Universe.
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- Even with all this astronomers, physics, and science with different measurements get results ranging from 70 km/sec/mpc to 62 km/sec/mpc. That is 15% apart with 10% error bars for each measurement. That is the best we can do today. There is a lot more to learn. So, the graduate student picked the right questions to make a career out of it.
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- To reduce the errors in measuring our distance ladder we need better rulers. Here is one example using the Hubble Space Telescope. Focused on a single galaxy, NGC4258 in Ursa Major Constellation there are clouds of water vapor emitting a single frequency spectrum in radio waves. The galaxy also contains Cepheid Variable stars. By tracking the motion of the radio observations to triangulate a distance that could match up with the Absolute Brightness of Cepheid’s astronomers calculate a distance of 23.5 lightyears.
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- Knowing the distance astronomers could calibrate the Absolute Brightness of the Cepheid’s. Knowing that they could calibrate the Absolute Brightness of a Type 1A supernova occurring in the same galaxy. Knowing that they can say the Universe is expanding at 73 km/sec/mpc with an error of less than 4.3% . Wow! It took a whole career to get to that number.
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- Request these Reviews to learn more:
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- Footnote (1) - Supernovae as Standard Candles. We use supernovae explosions as cosmic yardsticks to chart the expansion of the universe. If we know the Absolute Brightness of a light source we can calculate its distance by measuring its apparent brightness. We know mathematically the brightness falls off as the square of the distance light travels
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- To do this calculation we depend on each supernova explosion to be exactly the same. There is a particular “ Type 1A” White Dwarf supernova.
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- One type occurs in a binary system whereby a White Dwarf reaches a mass of 140% of the mass of our Sun where gravity overcomes the electrons degeneracy pressure. This limit is called the Chandrasekhar Limit after Subrahmanyan Chandrasekhar, an Indian astrophysicist who made this calculation in 1930 when he was only 19 years old.
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- The second type of Type 1A supernova explosion that can occur is when two White Dwarf stars collide. The explosion that results may look the same but the brightness could be much brighter. This would mean we have a different model for a “standard candle”, called the “ double-degenerate model”.
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- Stars collapsing with greater than 1.4 Solar Mass evolve into stellar remnants, directly into Neutron Stars or into Blackholes. White Dwarf stars exactly 1.4 Solar Mass explode before they undergo this collapse. White Dwarf stars under 1.4 Solar Mass simply remain White Dwarf stars.
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- Our sun will remain a White Dwarf star after it exhausts all of its nuclear hydrogen / helium fuel. That will occur 5 billion years from now. Of course, that assumes we do not collide with another star and become a “ double-degenerate “ supernova.
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----- 707-536-3272 ---------------- Friday, April 8, 2016 -----
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