How to measure the mass of galaxies and the universe, and how scientific thinking has to move from the simple to the seemingly complex

2020-06-19

The Universe and Science from Simplicity to Complexity

Cosmologists chart the timeline of the evolution of the universe, with the first stars born about 180 million years after the Big Bang

The universe is a constant source of fascination for humans, with exoplanets, shining stars, vast rivers of stars, and mysterious dark matter and dark energy. Human beings are full of dreams and imagination when they look at the universe, such as whether there are any civilizations in the universe that are on the same level as ours. Are you thinking the same thing? Is there life in the universe, how big is the universe, and so on. Thinking goes as far as the human imagination can go.

To understand more about the universe and explore the universe, dreams and imagination are not the most critical, we also need the help of science. Back to the essence of science, the origin of the universe, out of the imagination thinking, we will always have many seemingly simple but in fact the most fundamental questions. For example, how many stars and galaxies are there in the universe, and what is the relationship between black holes and dark matter and dark energy? What's beyond the universe, what was before the big bang, and so on. These questions may seem simple, but the answers are very complex, and there are very complex scientific variables involved. There are no absolutes in everything, everything is in flux.

All matter in the universe is distributed in the structure of the cosmic web, the illustration is an enlarged high-resolution image of the local cosmic web, which spans 100 million light years, and the empty structure of the cosmic web is composed of dark matter dark energy

Human science is still in its infancy, and as we explore the science of the universe, more questions will surface, and sometimes Seemingly simple questions can be more difficult to answer. Today we are going to explore the seemingly simple question of how to "weigh" different celestial structures. When astronomers make theoretical deductions, they always talk about how big the exoplanet is, how many galaxies it is, and how many solar masses it has. Wait, how do astronomers know or calculate the size of galaxies of planets and stars beyond our imagination? There is a science behind calculating the structure of a star or other celestial body.

Start by calculating the mass of a planet.

Before we can calculate the masses of other planets, we need to start by determining the mass of the Earth, and Newton's law of gravity tells us that the two The gravitational force between the objects is proportional to the product of the masses between the two objects, which can also be recorded as their masses. divided by the square of the distance between their centers of mass. To get a reasonable approximation, we can assume that their geographic center is their center of mass.

Knowing the mass of the Sun, we can calculate the masses of the other planets

Knowing the mass and radius of the Earth and its distance from the Sun, we can again use the laws of gravity to calculate the Sun's Mass. The gravitational force between the Earth and the Sun is the gravitational constant G times the mass of the Sun the mass of the Earth, divided by the distance between the Earth and the Sun the square. One thing to note is that the gravitational force must be equal to the centripetal force required to keep the Earth in a (nearly circular) orbit around the Sun, and the centripetal force is the Earth's The mass of the earth multiplied by its velocity divided by the square of the distance to the sun. By astronomically determining the distance to the Sun, we can calculate the speed of the Earth around the Sun and thus the mass of the Sun.

Once we have data on the Sun's mass, we can astronomically determine the orbital radii and periods of the other planets, calculate the required centripetal force, in which case we can determine the mass of any planet. In other words, the mass of a planet is determined by its gravitational effect on other bodies.

The true proportions of several exoplanets and primary stars to the Earth

In order to calculate the mass of another planet, we must somehow measure the strength of its gravitational pull on another object. If the planet has a natural moon, then this is much simpler, by observing the time it takes for the satellite to orbit its host planet. We can use Newton's equations to infer the mass of a planet. For planets that are undiscovered or have no natural moons at all, we'll need to take other approaches. Mercury and Venus, for example, have no moons, but they do exert a tiny gravitational pull on each other and the other planets in the solar system, and astronomers These small deviations can be used to determine the masses of these natural-satellite-free planets.

To date, scientists have confirmed that there are more than 4,000 exoplanets orbiting stars outside our solar system. To determine if these distant planets are habitable, it is necessary to know the mass of the exoplanet, knowing that the planet is very important because the data help astronomers infer whether the planet is made of gas or rock, in addition to the planet's makeup In addition, exoplanet mass data can also help astronomers argue that planetary surface and interior activity, such as plate tectonics, global data such as magnetic fields.

In total, astronomers and space telescopes have discovered more than 4,000 exoplanets, and that number will grow in the future as new exoplanet space telescope missions are launched

However, techniques for estimating exoplanet masses are currently limited, and radial velocity is the main method scientists use to calculate exoplanet masses. This method works by observing tiny wobbles in the orbit of a star, which appear because it is pulled by the gravity of the planet, and scientists The mass ratio of planets to stars can be derived from this. For large Neptune-sized planets or Earth-sized asteroids orbiting very bright stars, the calculation of radial velocities is relatively easier to carry out.

Last year, astronomers at MIT developed a new technique that uses only the lensing spectra of planets to determine the system's mass of an exoplanet. The principle of this transmission measurement spectrum is to measure the inclination of a planet's atmosphere in the transmitted stellar light as it passes through the star. These data can be used to determine the size and atmospheric properties of a planet, and can also reveal the mass of a planet.

Radial velocity and transmission spectroscopy are the two main methods used to determine the mass of exoplanets.

Astronomers can now use space telescopes and large ground-based telescopes to analyze the transmission spectra of exoplanets. When a planet passes in front of its star and some light passes through the planet's atmosphere, a transmission spectrum is created. By analyzing the wavelengths of the light, scientists can determine a planet's atmospheric properties, such as its temperature and the density of its atmospheric molecules. From the total amount of light that is blocked out, the size of the planet can be calculated.

To test this method, MIT astronomers used the technique to measure a system numbered HD189733b exoplanet, which lies 63 light-years away. Based on mass data from the transmission spectrum, astronomers have produced the same mass results as the radial velocity method. In the future, the specifications of high-resolution space telescopes will be upgraded, like the James Webb Space Telescope, which has an infrared Scientific instruments are a powerful tool for observing exoplanet atmospheres, and this new technology will be adapted to more space telescopes in the future, astronomer Mass data on exoplanets will be increasingly accurate.

Artistic rendering of transmission spectroscopy

Observing galaxies in the midst of it.

How many planets are there in the universe? No one can extrapolate the exact number, and an analogy is that there are more planets in the universe than there are sand on Earth. We don't know the number of planets in the universe, but scientists estimate the number of galactic planets to be between 200 and 400 billion. What is the mass of a galaxy made up of so many stars in a billion interval? It's a problem for astronomers. Using new data from NASA's Hubble Space Telescope and the European Space Agency's Gaia satellite, astronomers have recalculated the galaxy's mass, which results in our galaxy weighing about 1.54 trillion solar masses.

Why 1.54 trillion solar masses? First we need to look at the components of this value, first of all the 200 to 400 billion planets in the galaxy account for the mass of the A fraction, followed by the 4 million solar mass black hole at the center of the galaxy. Both parts are a small fraction of the galaxy's mass composition, and all that's left is what astronomers know about dark matter and dark energy, and the An estimate of the halo structure near the Milky Way.

There are also very large halo structures on the periphery of the Milky Way, which we can interpret as a kind of hot plasma, and which astronomers count within the mass of the galactic structure, the nature of which we are currently unable to determine.

Space telescopes can directly observe matter to better infer mass, and in the future, with multiple wide-field space telescope missions, the galaxy The number of planets in the middle will get more and more accurate, but dark matter and dark energy have always been a headache for astronomers. What is dark matter? We really don't know, and astronomers can only assume at this point that it could be piles of undetectable peculiar theoretical particles. Not only do dark matter and dark energy make up an extremely high percentage of the mass of all cosmic galaxies, even about 96% of the observable universe is dark matter! with dark energy composition.

Understanding the mass of our galaxy is important in astronomy, and without knowing the mass of the Milky Way, it is difficult to calculate how it is related to the Nearby galaxies, such as the Andromeda Galaxy, interact with each other. Understanding the mass of the Milky Way also helps us to better understand how it evolved, giving us a better understanding of how other galaxies formed. more understanding. In the future, astronomers hope to gain a more accurate understanding of the mass of the Milky Way in order to place it in the context of a theoretical cosmological framework of The precise mass of the Milky Way galaxy is a critical part of many cosmological questions when compared to data from galaxy simulations in the early universe.

All the stars we see at night are within the red circle.

Astronomers now have the technology to estimate the mass of distant galaxies using the speed at which they rotate, but measuring the mass of the Milky Way Much more difficult because we are inside the galaxy and can't understand its big picture. An example would be if you want to know how big your house is, but you can't leave your closet, you can only extrapolate from inside your closet The area of an entire, incredibly large house is extremely difficult.

And it's impossible to "weigh" a galaxy just by looking at it, let alone by the fact that the observer happens to be inside it, though. The larger and more massive a galaxy is, the faster its inner clusters move under the influence of gravity. So starting in January, astronomers are preparing to observe 157 globular clusters orbiting the center of the Milky Way, or the very A dense cluster of stars to calculate the speed of the galaxy.

Cluster of stars in the Abell 2744 galaxy

Astronomers have looked at 34 distant star clusters in preparation for a 22-month period using the European Space Agency's Gaia astrometric satellite The mass of these clusters has been estimated, and most of them are 6,500 to 70,000 light-years from Earth. Astronomers have also studied another 12 clusters observed by the Hubble Telescope, about 130,000 light years away. As the observations are continually updated, the motion of these galaxy clusters gives astronomers enough data to estimate the rotation speed of the galaxy as a whole. They can use this data to calculate the masses of galaxies.

To "weigh" the universe.

Stars, which make up galaxies, galaxies with supergiant structures, and which all include the cosmic void structures that make up the universe, about the mass of galaxies , astronomers can still have an interval value, an internal reference, but the mass of the universe scientifically speaking, we cannot be known. Because it is necessary to calculate not only the masses of all the planets and galaxies, but also the dark matter dark energy in interstellar space, dust clouds and even The mass of neutral hydrogen. Astronomers have been probing the value of the mass of the universe for more than a century, and they are still looking for ways to be more precise.

In February 2015, the Planck Cosmic Explorer research team released data that pinpointed the cosmic density energy value as 4.9% ordinary matter, 25.9% dark matter and 69.1% dark energy, and of the remaining baryonic matter, only one-tenth is dense

Measuring the mass of the universe is an important parameter for understanding its history and evolution, and while dark energy drives the expansion of the universe, baryonic visible matter will try to preventing the expansion of the universe, and that this counteracting force collectively makes up the average density of matter and energy in the universe, known as the cosmic density parameter. This parameter is crucial to the Standard Model of cosmology, and one way to measure it is to look at the cosmic microwave background CMB. explosion produces a small change in the temperature of the glow, and the values of these changes can tell us how fast the universe is expanding, which in turn will allow the We know the density of matter in the universe, which in turn infers the mass of the observable universe.

Another way to measure the mass of the universe is to observe how light from distant galaxies is deflected by galaxies, the gravitational lens effect, which is the A rough method of estimation. When astronomers make gravitational lens observations they will first compare individual galaxies and then astronomers will make statistical comparisons. Since we know the shape of most galaxies in the Universe, we can compare them to the shape of the lens we see in order to statistically How many lensing effects exist between galaxies and the Earth is the subject of a project by astronomers called the Thousand Degrees Survey.

A diagram of the gravitational lensing phenomenon, in which clusters of galaxies are so strongly gravitated that they bend the light of the galaxies behind them.

The lens effect can measure the mass between us and distant galaxies, but does not provide a value for the density of the universe, and for this reason astronomers also Need to know how far away galaxies are. So astronomers also determine the distance to the Milky Way by measuring their redshift values at several wavelengths, resulting in a cosmic The density parameters are slightly different from those calculated from the CMB. This is because in the Standard Model, the amount of dark energy in the universe is assumed to be constant, however, based on the latest data replacing the Standard Model. Dark energy changes over time.

There will be more space telescopes in the future to help us explore the universe and science for answers.

The important values behind seemingly simple questions

Behind some complex problems there are simple questions that correspond to them, and understanding galactic masses we can learn about the evolution of the Milky Way and even the The evolution of observable galaxies throughout the universe, and by looking at early galaxies and estimating their masses, we can learn about the early matter of the universe state energy state and so on. The knowledge of the mass of the universe will completely enrich the model of the universe, which will be extremely important for the theory of the evolution of the universe and the prediction of the future direction of the universe. Impact.

Science cannot be separated from computation, and scientific progress cannot be separated from imaginative thinking, and we need to think deeply even when we are faced with seemingly simple scientific problems, all the more so because science is made from simple to complex.


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