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-------------------- 2579 - UNIVERSE - how its age was determined?
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- The answers we get all depends on the measurements and how accurate astronomers can determine the expansion rate. And, then make the assumption is that it is a “good average” over prior accelerating and decelerating rates back to the beginning.
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- So how do they make the measurement of acceleration, which is velocity divided by time? Standard candles and standard rulers are two different techniques astronomers use to measure the expansion of space at various times / distances in the past. Based on how quantities like luminosity or angular size change with distance, we can infer the expansion history of the Universe. Using this “candle method” is part of the distance ladder, yielding 73 km / s / Mpc. Using the ruler is part of the “early signal method“, yielding 67 km / s / Mpc.
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- We need to make these metrics more familiar to the average reader. Kilometers per second we are pretty familiar with but we can convert that to even more familiar units. 74 kilometers per second is equivalent to 44 miles per second, or 49,300 miles per hour.
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- It is megaparsecs that is the unfamiliar term to most of us. A megaparsec is a distance measurement that astronomers use and one mega-parsec is equivalent to 3,260 lightyears distance.
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- So, we can convert the expansion rate of the Universe to 49,300 miles per hour per million light years distance. At a distance of 10 million lightyears the rest of the Universe is expanding away from us at 493,000 miles per hour. At 10 billion lightyears distance, or, 10,000 million lightyears the expansion rate is 493,000,000 miles per hour. That is 70% the speed of light. The speed of light is 670,633,500 miles per hour.
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- This astronomer’s expansion rate for the Universe of or 74 km / sec / mega parsec is called the “Hubble constant” of the rate of expansion . On the largest scales, the galaxies we find in the Universe obey a very simple relation between the two observable quantities of distance and redshift, where the farther away an object is from us, the greater its measured redshift, or the faster it is accelerating away from us.
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- “Redshift” is the amount of stretching of light wavelength as it travels though expanding space. Knowing the amount of redshift astronomers can calculate how long light has been traveling though expanding space.
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- As light wavelength stretches wider it shifts towards the red end of the light spectrum. If the wavelength narrowed for a light source traveling towards us it would have shifted toward the blue end of the light spectrum.
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- The recession speed that you would infer from a galaxy's redshift equals the distance to that galaxy multiplied by the Hubble constant. That constant has the same value for pretty much every galaxy we measure, particularly for galaxies within a few billion light-years from us. Even though there are additional cosmic motions inherent to each galaxy induced by gravitational effects, this law remains true when you average over all the galaxies you can find.
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- However when we get into the details what we measure the Hubble constant to be depends on how you measure it. If we measure it by using signals that were imprinted all the way back in the earliest stages of the Big Bang, you get a value for the Hubble constant of 67 km /s / Mpc, with an uncertainty of 1-2%,
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- If we measure it by measuring individual light sources that don't arrive until the Universe is already billions of years old, you obtain a value for the Hubble constant of 73 km / s /Mpc, with an uncertainty of just 2-3%.
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- Why do these two values not match, and why do they give such different, mutually inconsistent answers. Astronomers are currently working on this problem. But, either answer tells us a lot about what is happening.
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- You will notice something about the Hubble constant itself: it comes in units that are a speed (km/s) per unit distance (Mpc, where 1 megaparsec is about 3.26 million light-years). If you look at a galaxy that's 100 Mpc away, you'd expect it to recede away ten times faster than one only 10 Mpc away, but only one-tenth as fast as a galaxy 1,000 Mpc away. That's the simple power of the redshift-distance relationship.
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- There is another way to manipulate the Hubble constant. That is to recognize that a speed (distance-per-time) per (divided by) unit distance (distance) is the same as units of inverse time , reciprocal or time, (1 / time). What could the physical meaning of that "inverse time" correspond to? It could correspond to the “age of the Universe“.
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- There are approximately 3.1 × 10^19 kilometers in one megaparsec, which means that if you turn the Hubble constant into an inverse time, then the "time" that a value of 67 km / s / Mpc corresponds to is equivalent to 14.6 billion years, the age of the Universe.
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- The "time" that a value of 73 km / s / Mpc corresponds to is equivalent to 13.4 billion years.
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- These are both almost equal to the accepted age of the Universe, but not quite. In addition, they're both almost equal to one another, but differ by approximately the same amount that the two estimates for the Hubble constant differ by, that is about 9% .
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- The value of the Hubble constant today isn't simply the inverse of the value of the age of the Universe, even though the units work out to give you a measure of time. Instead, the expansion rate that you measure ,the Hubble constant today, must balance the sum total of every form of energy that contributes to the Universe's composition, including:
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--------------------------------- normal matter,
--------------------------------- dark matter,
--------------------------------- neutrinos,
--------------------------------- radiation,
--------------------------------- dark energy,
--------------------------------- spatial curvature,
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- The age of the Universe multiplied by the Hubble constant will equal different values for Universes made up of different compositions. If your Universe is exclusively made up of radiation, you find that the Hubble constant multiplied by the age of the Universe since the Big Bang equals ½, 50% exactly.
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- If your Universe is exclusively made up of matter (normal and/or dark), you find that the Hubble constant multiplied by the age of the Universe equals ⅔, 67% exactly.
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- And if your Universe is entirely made of dark energy, you'll find that there is no exact answer; the value of the Hubble constant multiplied by the age of the Universe always continues to increase (towards infinity) as time goes on.
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- This means that if we want to accurately calculate the age of the Universe, but the Hubble constant alone isn't enough. In addition, we also need to know what the Universe is made out of. Two imagined Universes with the same expansion rate today but made out of different forms of energy will have different expansion histories and, therefore, different ages from one another.
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- Measuring back in time and distance can inform how the Universe will evolve and accelerate / decelerate far into the future. We can learn that acceleration turned on about 7.8 billion years ago with the current data, but also learn that the models of the Universe without dark energy have either Hubble constants that are too low or ages that are too young to match with observations.
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- If dark energy evolves with time, either strengthening or weakening, we will have to revise our present picture. This relationship enables us to determine what's in the Universe by measuring its expansion history.
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- So, to find out how old the Universe actually is since the onset of the hot Big Bang, all we have to do is determine the expansion rate of the Universe and what the Universe is made out of.
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- There are a variety of methods that we can use to make this determination, but there's one vital thing we have to remember: many of the ways we have of measuring one parameter (like the expansion rate) are dependent on our assumptions about what the Universe is made out of.
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- In other words, we cannot assume that the Universe is made out of a certain amount of matter, a certain amount of radiation, and a certain amount of dark energy in a way that's independent of the expansion rate itself. Perhaps the most powerful way to illustrate this is to look at the leftover glow from the Big Bang itself: the Cosmic Microwave Background.
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- The leftover glow from the Big Bang, the CMB, isn't uniform, but has tiny imperfections and temperature fluctuations on the scale of a few hundred micro-Kelvin. While this plays a big role at late times, after gravitational growth, it's important to remember that the early Universe, and the large-scale Universe today, is only non-uniform at a level that's less than 0.01%.
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- The Planck satellite has detected and measured these fluctuations to better precision than ever before, and can use the fluctuation patterns that arise to place constraints on the Universe's expansion rate and composition.
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- Overall, every direction in the Universe displays the same average temperature as every other direction: approximately 2.725 Kelvin.
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- The temperature fluctuations exhibit particular patterns in their magnitude on a variety of angular scales, with the fluctuations rising in magnitude down to some particular angular scale of about 1 arc-degree, then decreasing and increasing in an oscillatory fashion. Those oscillations tell us some vital statistics about the Universe.
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- Four different cosmologies lead to the same fluctuation patterns in the CMB.
Given the fluctuations we see, we can have a Universe with these possibilities:
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- 4% normal matter, 21% dark matter, 75% dark energy and a Hubble constant of 72,
- 5% normal matter, 30% dark matter, 65% dark energy and a Hubble constant of 65,
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- 8% normal matter, 47% dark matter, 49% dark energy, -4% curvature and a Hubble constant of 51.
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- You will notice a pattern here: you can have a larger Hubble constant if you have less matter and more dark energy, or a smaller Hubble constant if you have more matter and less dark energy. What's remarkable about these combinations, however, is that they all lead to almost exactly the same age for the Universe since the Big Bang.
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- There are many possible ways to fit the data that tells us what the Universe is made of and how quickly it's expanding, but these combinations all have one thing in common: they all lead to a Universe that's the same age, as a faster-expanding Universe must have more dark energy and less matter, while a slower-expanding Universe requires less dark energy and greater amounts of matter.
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- The reason that we can claim the Universe is 13.8 billion years old to such enormous precision is driven by the full suite of all the data that we have. A Universe that expands more quickly needs to have less matter and more dark energy, and its Hubble constant multiplied by the age of the Universe will have a larger value.
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- A slower-expanding Universe requires more matter and less dark energy, and its Hubble constant multiplied by the age of the Universe gets a smaller value.
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- However, in order to be consistent with what we observe, the Universe can be no younger than 13.6 billion years and no older than 14.0 billion years, to more than 95% confidence.
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- There are many properties of the Universe that are indeed in doubt, but its age isn't one of them. At my age I don’t even buy green bananas. Here are some other ages you might find interesting:
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--------------------- Age of the Milky way galaxy --------------- 10 billion years
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---------------------- Age of the Earth ----------------------------- 4.50 billion years
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---------------------- Age of the Sun -------------------------------- 4.51 billion years
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---------------------- Age of the author ------------------------------ 0.000,000,079 years
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- January 8, 2019 2579
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--------------------- Thursday, January 9, 2020 --------------------
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