What is the baryon acoustic oscillation? Why can it be called the "standard bar" of the universe?

2020-01-16 | Quantum Science original |

Baryon acoustic oscillations are the regular periodic density fluctuations of baryon matter visible in the universe. Just as supernova can be used as standard candlelight, the mass clustering of baryon acoustic oscillations can also be used as a standard ruler for measuring cosmological distance.

The baryon acoustic oscillation name sounds high-end, in fact, it is a scientific measurement method to understand the history of the expansion of the universe! Learn what it is today?

Imagine that you are observing the universe. In the universe you will see some light spots, including planets, stars, galaxies, galaxy clusters, etc., if you want to use what you see to measure the universe from the past to the presentHow it swells every moment.

What would you do?

How do we measure and discover the expansion history of the universe

Every object in the universe has some inherent properties, that is, the physical characteristics of the object itself. This includes :

its quality,

its size,

and its luminosity or intrinsic brightness.

If our observation equipment is good enough, we can directly measure the apparent size or apparent brightness of an object, that is, how big or bright the target object looks from the position of our earth.

And some target objects in the universe have their intrinsic properties. For example, if we observe a star or galaxy, then its essential properties can also be easily measured, such as the width and luminosity of its emission line.The period of change, or the shape of its light curve! This tells us the nature of the object we see.

If we can do the following three things :

know the inherent properties of an object, internal brightness

Measure the same apparent characteristics of the object, apparent brightness

Measured distance or receding speed / redshift, Doppler effect

We can understand how the universe collided in its history! Astronomers have mastered two methods to measure the expansion of the universe.

One is to use brightness as a standard to measure: if we know how bright something is in nature, and then measure its apparent brightness, we also know how brightness moves with distance and red shift in the expanding universe Changes, you can infer the expansion history of the universe in this way. When we use brightness to measure, the object used is called standard candlelight, because if we know the inherent brightness of the candle, we only need to measure how it looksHow bright it is, you can immediately know how far it is from us. This is an example of Hubble using Cepheid variable stars and later people using type Ia supernova.

Another method is to use the size of the object: if we know how big an object is in nature, we can measure how big it looks its angular size, and we also know how the size is in the expanding universeAs distance and redshift changes, you can see how the universe evolved into what it is today.

Using such a physical size is called a standard scale, but the only object that is "standardized" is that the size of a single star is too small. Galaxies do not have a standard size.

The above are the two methods we used before. But the first one is more familiar to everyone.

How Baryton Acoustic Oscillation Reveals the Expansion History of the Universe

When we learned what the universe is made of, especially when we learned about the existence of dark matter and the inflationary period before the Big Bang, everything changed. We know that the distribution of matter was almost uniform at the beginning of the universe, There are slight fluctuations on all scales, or areas where the density of matter is slightly larger or slightly smaller than the average density.

With the age of the universe, gravity moving at the speed of light can reach farther and farther, causing more and more scales to shrink and collapse. When the universe was still young, the gas cloud did not cool in place The temperature is still high. If the gas cloud collapses too much, the radiation pressure will push the gas cloud out again. Gravity will attract some gas clouds to collapse again. If the temperature is still high, the radiation pressure will be againThe gas cloud is pushed out again. Since the gas cloud is baryon material, this back and forth oscillation is similar to sound waves, so we call it baryon acoustic oscillation.

This is why we will see some swinging and undulating patterns in the afterglow of the Big Bang.

Over time, as the universe expands, the first large fluctuation peak will be converted to a scale. At this scale, we are more likely to see two galaxies spaced a certain distance away. Today this distance is quite similarAt 500 million light years, this means that if we choose a galaxy in the universe, we are more likely to find a second galaxy at a distance of 500 million light years, rather than at 400 million light years or 600 million light years.

This kind of distance scale the interconnected scale of galaxies is called the acoustic scale, because it is the baryons such as protons that oscillate back and forth in these areas with high density. The phenomenon that causes this distance correlation is called baryonsAcoustic Oscillation BAO, we can use this redshift to measure changes in the expansion rate of the universe over time.

Just 20 years ago, baryon sound oscillations were not a viable method for measuring anything in the universe. But with the two-degree field galaxy redshift measurement 2dFGRS and the current Sloan Digital Sky Survey SDSS and other measurement methodsIt appears that we have measured enough galaxy positions and redshifts, and obtained unprecedented details of the large-scale structure of the universe.

Data released by SDSS-III, this is a galaxy map. Each point and pixel on the picture represents the entire galaxy.

By baryon sound wave oscillation, what we get from it is not only that dark energy accounts for two-thirds of the total energy of the universe consistent with the cosmic microwave background radiation and supernova data but also that dark energy is consistent with the cosmic constant, which will remain constant over time.Change to the highest accuracy ever!

Ten years ago, we knew that the universe was dominated by dark energy, but the uncertainty of w, that is, the certainty of the dark energy equation of state parameter w = P / ρ is very large. We can say that w is in the range of -0.5 to -3.0This is a large range. Today, due to the acoustic oscillations of baryons, we can say that w is between -0.87 and -1.15, which is an incredible progress! Future investigations, such as those to be conducted by LSST, willReduce this uncertainty to a few percentage points: if it goes well, we should be able to say that w is between -0.98 and -1.03.

Summary: So, what exactly is a baryon acoustic oscillation?

The universe begins with tiny fluctuations in density. Gravity acts on ordinary matter and dark matter at the same time, but only ordinary matter is pushed out through electromagnetic interactions. This fact creates a "special scale" in the universe. Today, we can passObserve the distance between galaxies to observe this particular scale, and this distance evolves with the expansion of the universe and over time.

Not only today, but on all the scales we can measure, understand the entire history of expansion of the universe.

This is a way to understand the composition of the universe, the history of expansion, including dark energy, without the need to know the brightness of any object!

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