1. Why is the night sky dark?
This is about the so-called photometric paradox, or Olbers’ paradox, named after the 19th-century astronomer Heinrich Wilhelm Olbers, who popularized it. The paradox has been known since at least the 16th century. If the Universe is infinitely vast, it should contain an infinite number of stars, and thus, in any direction we look, our gaze should encounter the surface of a star. Although distant stars appear fainter, there are far more of them. Therefore, due to the cumulative radiation of an infinite number of stars, the entire sky should shine like the Sun. Obviously, this is not the case. But then why is the night sky dark? The answer may lie in the finiteness of the age of the Universe or the finite number of stars it contains. In the early 20th century, Lord Kelvin (William Thomson) suggested that it’s because the light from sufficiently distant stars has not yet reached us due to the finite age of the Universe. This implies that the part of the Universe we can see, the observable Universe, is not infinite and therefore cannot contain an infinite number of stars. This is the solution to the paradox. According to modern data, more than 13 billion years ago, there were no stars in the young Universe, so they cannot shine on us from that distance. The expansion of the Universe also contributes to the dimness of the night sky due to redshift.
2. How cold is space?
It depends on where the measurements are taken. In direct sunlight near a celestial body, it can be very hot. For example, a spacecraft near Mercury can heat up to 300°C on one side while cooling to -200°C on the other side facing open space. This is because there is no atmosphere to redistribute heat. Space vacuum is the best insulator. For a long time, it seemed meaningless to talk about the temperature of space itself, but this is not the case. Space is filled with relic radiation – microwave background – which heats cosmic gas to a minimum temperature of 2.7 Kelvin. Thus, the interaction of atoms and molecules with this relic radiation raises the temperature a few degrees above absolute zero (-273°C).
3. What makes a planet a planet?
According to the resolution of the International Astronomical Union, an object can be considered a planet if it: orbits around the Sun, is massive enough to acquire a spherical shape, and has sufficient gravitational dominance to “clear its orbit” of other debris. It was the latter, most contentious criterion that led to Pluto being “demoted” to a dwarf planet status in August 2006. This was the first attempt in history by astronomers to officially define what a planet is. The accepted definition implies that a planet must be significantly larger than any other objects with a similar orbit and gravitationally dominate its surroundings. Neptune holds Pluto and many other celestial bodies on close orbits. But Pluto is unable to gravitationally dominate its orbit and therefore, according to the definition, cannot be considered a planet.
4. How far in advance can polar lights be predicted?
Polar lights are the result of upper atmospheric glow, bombarded by charged particles from the solar wind. Spectacular auroras occur during solar storms caused by coronal mass ejections on the Sun. It takes charged particles from 18 to 36 hours to cover the 150 million km that separates us. This is the time frame for predicting “space weather”. To make an accurate forecast, a coronal mass ejection directed towards us needs to be registered, and then its speed measured. Several space probes, including SOHO, STEREO, and SDO, constantly monitor the Sun.
5. How hot are falling stars?
Meteors, commonly known as falling stars, are small rocks or pieces of iron entering Earth’s atmosphere at speeds ranging from 10 to 70 km/s, about 30,230 times faster than a bullet. At such speeds, they compress the air in front of them, heating it up to 2000°C. The hot air melts the meteoroid, creating a glow in the sky. The heat doesn’t penetrate into the stone because the heating only lasts for a few seconds. A meteorite that falls to Earth will most likely be cold.
6. Is the expansion of the universe accelerating?
It seems that indeed it is. Universal acceleration is one of the greatest mysteries of modern cosmology. It was discovered in 1998 when two independent groups of astronomers announced that supernovae, exploding in the distant cosmos, appeared dimmer than they should be according to their calculated distances. The only way to explain these observations was to assume that the universe is expanding faster and faster. The cause of this acceleration could be some form of energy, an undiscovered force of nature, or an unknown property of gravity. There is also a possibility that it’s just a “cosmic mirage” caused by simplifications in calculations based on the general theory of relativity. Whatever the explanation, our modern understanding of physics requires careful verification.
7. Is it true that we are all made of stardust?
Yes. All the iron atoms in your blood, calcium in your bones, oxygen in your lungs, carbon, nitrogen, phosphorus, sulfur, and so on, are much older than the Earth itself and were once part of giant stars. After the birth of the Universe, it contained only hydrogen and helium. These two elements make up 98% of all matter in space. Only 2% were processed into all other chemical elements found in nature. This transformation requires the temperature and pressure typical of stellar interiors, or even supernova explosions. At the end of a star’s life, these elements are ejected back into space. So all the atoms in our bodies, except for hydrogen, were once in the depths of stars.
8. Is our Universe part of the Multiverse?
According to several scientific concepts, our Universe may be just one of many – possibly infinite – other universes scattered in other dimensions of space and time. The so-called inflation theory claims that immediately after the birth of the Universe, space underwent a colossal expansion due to fluctuations in energy, which once started, can hardly stop. Not only did our Universe expand, but also countless other universes in a chain reaction that continues to this day. These other universes branched off from ours and are now completely unobservable to us. And from them, new universes can branch off in an infinite cascade. The idea of multiple universes unexpectedly emerged again in attempts to construct a theory that links all the forces of nature. Reality in this theory is described by space with 11 dimensions, of which we are familiar with only three. This construction, called M-theory, describes the landscape of all mathematically possible universes. Our own Universe is just one of the “valleys” of this landscape. Other “valleys” represent other universes, possibly obeying different laws of physics. These M-theory universes are separated from us but may influence us through their gravity. Although these ideas are still highly speculative, the Large Hadron Collider in Switzerland is already searching for signs of extra dimensions, and the ESA’s Planck satellite will try to find signs of inflation. If they succeed in finding something, the concept of multiple universes may be strengthened.
9. What is the age of the Solar System?
The Solar System consists of the Sun, planets, their moons, asteroids, and comets. It is believed that they all formed at the same time about 4.6 billion years ago. This conclusion comes from radioactive dating of uranium and other elements in Earth’s rocks and meteorites. All methods give dates close to 4.6 billion years. Previously, astronomers believed that the age of the Solar System was only tens of millions of years. These estimates were based on erroneous ideas about the reasons for the Sun’s glow and contradicted the data of geologists and biologists. Geologists believed that more time was needed to form Earth’s landscapes, and from a biological point of view, natural selection required more time to generate all the diversity of modern life. Both turned out to be correct. The question was finally resolved by radioactive dating in the 1950s.
10. Can a new Sun form by the time ours dies?
Stars in the Galaxy are constantly forming. On average, every year, material equivalent to the mass of our Sun turns into stars. The masses of stars vary from 0.1 solar masses for the smallest red dwarfs to 150 solar masses for the largest blue giants. Therefore, they are not born one by one per year. Nevertheless, there will always be plenty of stars in the Galaxy. But they are all far away, so the question is better posed as: can we devise a way to reach them within a reasonable time frame?
11. What is the largest planet discovered?
The largest known planet in the Universe, discovered by scientists, is called TrES-4b. This exoplanet, detected in 2006, has a mass about 1.7 times that of Jupiter. TrES-4b belongs to the class of hot Jupiters and is located at a tremendous distance from its star.
12. Are there places in the Universe with different laws of physics?
The laws of physics are considered immutable. That’s why we can observe the vast expanses of space and make sense of the results. If the laws of physics varied from place to place, all observations would be meaningless — it would be like four people playing the same board game with different rules.
Nevertheless, some observations suggest that although the laws of physics are constant, the values of fundamental constants may have slightly changed over the cosmic history. These numbers define basic physical characteristics such as the speed of light or the strength of gravitational interaction between objects (via the gravitational constant G in the law of universal gravitation). By observing how gas clouds absorb ultraviolet radiation from distant galaxies called quasars, some astronomers conclude that the set of electron orbits around the atomic nucleus changes slightly over time. This set is determined by the fine-structure constant, which is a combination of other constants, including the speed of light.
So, it’s possible that in the young Universe, the speed of light was higher, but it quickly decreased to almost its modern value. And in distant gas clouds, astronomers observe the final moments of this decrease. However, this conclusion remains highly controversial.
Other researchers are looking into changes in the gravitational constant G, as this could lead to a new understanding of gravity. However, there have been no reliable confirmations in this area yet.
13. What causes the largest explosions in the universe?
The energy released in gamma-ray bursts allows them to be detected even when they occur in the most distant regions of the Universe. In a few seconds, so-called short gamma-ray bursts emit as much energy as the Sun emits in its entire lifetime. Short gamma-ray bursts are apparently associated with the merger of neutron stars, while each “long” gamma-ray burst indicates the death of an extremely massive and rapidly rotating star, a hypernova, “collapsing” into a black hole with the formation of two “jets” – streams of matter moving at relativistic speeds. Earth is hit by one of these jets when we observe a gamma-ray burst. A nearby gamma-ray burst threatens the extinction of all life.
14. Why doesn’t the Moon fall onto the Earth?
This was answered by Robert Hooke and Isaac Newton. Although gravity pulls the Moon towards the Earth, the Moon’s motion is directed perpendicular to the force of gravity. It’s similar to how water is retained in a bucket being swung around on a rope: stop the rotation, and the water-filled bucket will fall. But in the frictionless environment of space, the Moon doesn’t slow down its orbital motion and completes one orbit around us every month.
15. What is dark energy?
Dark energy is a hypothetical substance proposed to explain the constant acceleration in the expansion of the Universe. Although cosmic acceleration was only discovered in the mid-1990s, Albert Einstein included a lambda term in his general theory of relativity, corresponding to one of the current concepts of dark energy. He called this term the cosmological constant and used it to represent the energy contained in the cosmic vacuum. Lambda quickly “fell out of fashion,” and in the mid-20th century, astronomers saw no need for it, and modern extensions of quantum physics explicitly exclude its possibility. But now astronomers want to bring it back. Someone clearly got something wrong somewhere.
16. What is dark matter?
Dark matter is a collection of hypothetical particles acting as gravitational glue, preserving the integrity of galaxies. The fact is that galaxies rotate so fast that they should have flown apart, as they evidently contain insufficient matter to generate a strong cohesive gravitational field. Even if all the stars, planets, gas clouds, and other celestial bodies were added together, galaxies still fall far short in mass. Therefore, astronomers and particle physicists have calculated that there must be particles whose nature remains unknown. They should provide galaxies with the missing gravity and help physicists expand the quantum theory.
17. Is there dark matter in our Galaxy?
Most astronomers believe that in our Galaxy, the ratio of dark to ordinary matter is about 10:1. It is believed that the concentration of dark matter increases as one approaches the center of the Galaxy, and a much more sparse gigantic halo of it surrounds the entire array of Milky Way’s luminous stars. However, no one has caught a single dark matter particle yet. Many experiments have already reached the sensitivity theoretically sufficient for their detection to become possible. So skepticism is gradually growing. The alternative is that we do not understand gravity as well as we think, and under certain circumstances, it can attract much more strongly than we think.
18. Can there be life made of dark matter?
This is extremely unlikely because most plausible candidates for dark matter cannot form atoms or combine into molecules. They are simply remnants of the Big Bang, floating in space and generating gravitational forces. But this statement is based on our current understanding of dark matter. There is the idea of a certain mirror matter that can form mirror atoms, forming mirror stars, planets, and life. If mirror matter indeed exists, then a whole mirror Universe may invisibly coexist with ours. Its only manifestation for us would be gravitational attraction.
19. What is a black hole?
A black hole is an object with such a powerful gravitational field that not even light can escape from it. Every black hole is characterized by the presence of an event horizon (the boundary of the black hole) containing a hidden singularity inside, which is separated from the rest of the Universe by this horizon. The existence of black holes follows from Einstein’s equations, and the first simplest solution was obtained by the German physicist Schwarzschild (Karl Schwarzschild) in 1915.
20. What is the shape of a black hole?
A classical black hole has a spherical shape (this is the shape of the event horizon itself). However, most real black holes should rotate, involving the spacetime continuum in motion, similar to honey swirling around a stirring spoon. Therefore, instead of the solution found by Schwarzschild for non-rotating and uncharged black holes, it is necessary to consider a more general solution called the Kerr-Newman solution, obtained in 1965. Within its framework, a black hole can have a different shape. The possibility of the existence of a “naked singularity,” covered by an event horizon, is also discussed. And within the framework of modern theories such as superstring theory or M-theory, a black hole can have the most bizarre forms—a disk, a closed spiral, or even rings, similar to those encircling Saturn. But all this is still nothing more than a mathematical abstraction.
Around the event horizon of a rotating black hole, there should also exist a region called the ergosphere. A hypothetical stationary observer located there cannot remain motionless and must rotate with a positive angular velocity (in the direction of the black hole’s rotation).
21. What happens to light in a black hole?
Like any object that enters a black hole, light will be completely absorbed and most likely, forever. A beam of light that passes close to a black hole but very close to it will be deflected and set on a new course.
22. Can anything escape from a black hole?
For a long time, it was believed that neither matter, nor light, nor information could leave the confines of a black hole. However, this is true only if quantum effects are not taken into account. In quantum mechanics, there is a possibility of overcoming otherwise insurmountable barriers. In 1974, British physicist Stephen Hawking predicted the emission of elementary particles by a black hole, predominantly photons, called Hawking radiation.
The physical vacuum is constantly filled with “virtual particles” that are being born and disappearing. The “boiling of the vacuum” near the event horizon of a black hole leads to the fact that sometimes one of the pair of newly born particles falls into the black hole, while the other rushes outward, thereby generating radiation and reducing the total energy of the black hole, which ultimately may simply evaporate.
The question of the possibility of obtaining information from beneath the event horizon has long remained controversial. However, in August 2004, at the International Conference on General Theory of Relativity and Cosmology in Dublin, Hawking presented a report indicating that although a black hole distorts the “swallowed” information, it does not completely destroy it and eventually emits it outward during the process of evaporation.
23. What happens at the center of a black hole?
We don’t know what happens at the center of a black hole. It is commonly said that there is a point called the singularity (in some solutions for a black hole, it may turn out to be a loop). Our modern theories cease to work there because they are not suitable for infinitely dense objects.
24. Will we ever know what is at the center of a black hole?
The laws of physics, as we know them today, do not work at the center of a black hole. Therefore, physicists are trying to develop new theories of gravity that answer questions about what is hidden there and where one can go from this very “hole.”
25. Do black holes explode?
If Stephen Hawking is correct, then black holes should gradually evaporate. Losing more and more mass, they evaporate faster and faster until they disappear in a flash of gamma radiation. But so far, no one has observed such an event.
26. How can we find out what planets outside the Solar System look like?
Much of what you’ve heard about exoplanets, planets outside the Solar System, is speculation based on estimates of size and likely surface temperature. The existence of many discovered exoplanets is indicated by their gravitational interaction with parent stars, causing the stars to slightly shift as the planets orbit around them.
The only way to find out what they are like is to analyze the reflected starlight from the planets. This has been done for several exoplanets, and on HD 189733b, water vapor, carbon dioxide, and methane were discovered.
The problem is that the star usually overwhelms the instruments — the light from the planet is drowned out by the star’s radiation. Astronomers use various methods to block out the starlight, but with limited success so far. Systematic determination of exoplanet properties is likely to become a reality only in 10-20 years.
27. Why do stars shine?
Stars shine primarily due to nuclear fusion reactions, in which hydrogen is converted into helium. Thus, every twinkling point in the night sky is a colossal nuclear reactor. In the core of each star, the density, temperature, and pressure of the gas are so great that atomic nuclei come close enough together to start the fusion process.
The energy released gradually moves outward, to the surface of the star, where it is eventually emitted as light. It is estimated that it takes tens or hundreds of thousands of years for light to travel this difficult path to the surface and freedom.
In white dwarfs, the outer layers of the star are stripped into space, and the exposed “core” of the reactor can be seen from the outside. It emits destructive ultraviolet light until it finally cools down and fades away.
28. How many dimensions exist?
Three spatial coordinates are available to everyone. The fourth coordinate can be considered time. However, according to Einstein’s concept, speaking separately about three-dimensional space and time is incorrect; the real world is a unified four-dimensional spacetime continuum.
Even more surprising are modern physical theories. Physicists increasingly talk about 11 or 12 dimensions. String theory suggests replacing point-like fundamental particles with vibrating segments of subatomic “strings” in “compactified” (folded) dimensions. Unlike the familiar three dimensions, compactified ones are tiny in size, making them extremely difficult to detect.
29. Are there undiscovered chemical elements in space?
Probably not — heavy elements tend to undergo radioactive decay and are short-lived. Most elements in nature are stable, having existed before the formation of the Earth and lasting billions of years in an unchanged form. They fill all positions in the periodic table. There are no heavy elements that are synthesized and then immediately decayed.
30. Everything in the world is made of something, but what is gravity made of?
The most ambitious physicists lose sleep over this question. There are four fundamental forces: electromagnetic, which gives us light, electricity, and magnetism; strong, or nuclear, which binds atomic nuclei together; weak, which governs certain types of radioactive decay; and gravity.
All forces except gravity are explained by quantum theory, where interactions are mediated by ephemeral particles. Physicists hope that gravity will also obey quantum laws and be carried by particles, which have been dubbed gravitons. The problem is that the mathematics becomes monstrously complex, and so far, there is no quantum theory of gravity that can be tested.
The best existing explanation of gravity is not quantum. Albert Einstein’s General Theory of Relativity describes gravity as the curvature of spacetime, a deformation of the “fabric” of space. Celestial bodies are placed on this “fabric.” Due to its deformability, it bends under each massive object, creating the famous spacetime curvature. These curvatures occur in the fourth dimension of space, which we, being three-dimensional beings, cannot directly comprehend. Instead, we feel a force pulling us toward the curvature, as if compelling us to roll downhill.
According to Einstein, gravity is not a force but a consequence of living in a universe with more dimensions than we can perceive. Nevertheless, Einstein’s theory is incomplete, as it breaks down in particularly strong gravitational fields — inside black holes and at the moment of the universe’s birth. So, a new theory of gravity is needed, and we simply haven’t found it yet.
31. How far into space can telescopes see?
The maximum distance any telescope can see is about 13.7 billion light-years. From a distance just under 13.7 billion light-years away, cosmic microwave background radiation reaches us — all that remains of the hot material released during the formation of the universe.
It creates an impenetrable barrier beyond which ordinary telescopes cannot see anything. In the future, detectors capable of capturing ghostly particles called neutrinos might penetrate beyond the boundary from which microwave background radiation comes.
32. How do discoveries of new planets change our understanding?
There are already over 500 planets beyond the solar system. If these exoplanets have taught us anything, it’s to be prepared for any surprises. Before the first discovery announced in 1995, astronomers mostly believed that other planetary systems should resemble our own, with rocky planets close to the star and gas giants farther away. This myth was shattered by the first planet found around a “normal” star (a main-sequence star). The mass of the newly found planet (51 Pegasi b) turned out to be half the mass of Jupiter, but it was so close to its star that it orbited it in just four days. For comparison, tiny Mercury takes a lengthy 88 days to orbit the Sun. Subsequently, many similar “hot Jupiters” have been found.
It is believed that such large planets form in the cold outer regions of planetary systems, beyond the so-called ice line. Here, the temperature, maintained by the central star, drops significantly below the freezing point of water, methane, and ammonia, which form solid particles. They, mixed with rocky and metallic debris, provide rapid mass growth for the planet. As the planet moves through the remnants of the dust disk from which it formed, its resistance robs it of energy, causing it to spiral inward toward the center of the system. Eventually, the planet ends up close to the central star.
There are also other anomalous exoplanets: Wasp 12 b, which might have mountains made of diamond and graphite; CoRoT-7 b, where oceans of molten lava might exist; and HD 209458 b, which is evaporating due to the heat of its star with potentially dangerous trajectories. The greatest threat is posed by the 270-meter asteroid Apophis. When it was discovered in 2004, it seemed that it might collide with Earth in 2029 or 2036. Further observations refined the parameters of its orbit, and fortunately, we now know that the chance of a collision with Earth in those years is infinitesimally small.
33. What determines the orbits of planets?
They are determined by the force of gravitational attraction acting on them, which depends on the distance between the planet and the central star, as well as the mass of the star and the planet itself. This law holds true throughout the universe. Since stars and planets are not points but extended objects, different parts of them are attracted differently. In a non-uniform gravitational field, tidal forces act on an extended object. The closer the bodies are to each other, the stronger these forces are. They lead, among other things, to the fact that the Earth’s own rotation is slowed down by the Moon, and the Moon moves away from the Earth.
34. Can anything survive on a meteorite?
It is extremely difficult for life to survive on a meteorite. To shield itself from X-ray and other lethal cosmic radiation, it must be deep within the rock. Furthermore, there would be very little food for life there. Although some bacteria on Earth can turn into spores ready to come to life with the first drops of water, for these bacteria to survive on a meteorite, they would have to endure for hundreds of thousands of years. However, a typical interstellar journey would stretch over millions or even billions of years. Meteorites from the Moon and Mars have already been found on Earth, so in principle, such journeys are possible. In 1984, NASA scientists suggested that they had found fossilized bacteria in a Martian meteorite they had studied. However, this claim remains doubtful for now.
35. Can an asteroid collide with Earth?
Many asteroids have collided with Earth in the past, and such events may occur in the future, but it is a rare occurrence. The last such collision occurred in 1908 when a bolide devastated an area in Eastern Siberia equivalent in size to London. Fortunately, no one was killed in that incident. Today, robotic telescopes constantly scan the sky for asteroids with potentially hazardous trajectories. The greatest threat is posed by the 270-meter asteroid Apophis. When it was discovered in 2004, there was initially concern that it could collide with Earth in 2029 or 2036. However, further observations have refined its orbit parameters, and fortunately, we now know that the chance of a collision with Earth in those years is extremely small.
36. What was before the Big Bang?
Some scientists believe that time itself emerged only in the Big Bang, so there can be no “before.” Others think that before the explosion, the Universe was in some passive state. A third view is that the Big Bang ripped the world out of a temporal loop, allowing time to move forward and the Universe to expand.
37. What caused the Big Bang?
According to the so-called ekpyrotic scenario (the “igniting universe” model), the Big Bang was caused by the collision of two branes in a cold, multidimensional Universe, with subsequent energy release on one of the branes, which we perceive as the known Universe. This idea suggests that we are part of the Multiverse (a whole cluster of parallel universes).
38. Why is the Universe expanding?
The simple answer: because it is expanding. Formulating the general theory of relativity, Einstein revealed that an inherent property of space is that it must either expand or contract. He found this incredible until Edwin Hubble discovered that galaxies are moving apart in space, carried away by its expansion.
39. How will the existence of the Universe end?
Most scientists believe that the Universe will continue to expand forever, and galaxies will drift farther and farther apart. This is especially true if dark energy exists, accelerating the expansion. In an especially dangerous scenario, dark energy could tear everything apart, including atoms (the Big Rip).
40. If there was a Big Crunch, what caused it?
The Big Crunch could have occurred if there was enough mass to reverse the expansion of space. The gravitational pull of all celestial bodies would bring them all together. The result would be a cataclysm in which all celestial bodies in the Universe collided. Some believe this could have triggered a new Big Bang.
41. Have we ever been precisely in this place in space?
Almost certainly not. If there’s anything constant in the Universe, it’s motion. Everything is moving relative to everything else because every cosmic object has mass, generating gravity that pulls all other objects toward it. Earth is constantly orbiting the Sun, the Sun is constantly moving around the center of the Galaxy, which itself is being pulled towards the Andromeda Galaxy, and this pair is falling towards the Virgo Cluster, which is 60 million light-years away from us.
42. If other universes exist, could they be of a different color?
The colors we see are the result of our brain’s perception of a narrow range of electromagnetic wave frequencies. The wavelengths of light visible to us range from approximately 350 to 700 nanometers. The shortest wavelengths correspond to the color violet, and the longest to red. The frequencies our eyes perceive are determined by pigments in the retina. If we were equipped with different pigments sensitive to different frequencies, we could see, for example, in the infrared range, but nobody knows how that color would be perceived.
43.How are planets formed?
Planets are essentially a byproduct of star formation. Stars are born from giant molecular clouds as a result of gravitational collapse and fragmentation. During this collapse, the cloud begins to rotate faster, leading to the formation of a disk of material around the equator of the young star. Inside this disk, planets are born.
Planets begin to form from dust grains that collide and stick together, forming planetesimals. Finally, these planetesimals collide and merge to form a mature planetary system. It’s likely that in our Solar System, there were initially at least 50 small rocky planets that merged to eventually form Mercury, Venus, Earth, and Mars.
44. What shape is the Universe?
It’s like with Earth: it appears flat to us, but from large-scale observations, we know we live on a sphere. The shape of the Universe depends on the total amount of matter and energy it contains. They curve the space-time continuum, turning it into something resembling either a sphere or a saddle. The extremely unlikely third possibility is that the Universe is flat, like a sheet. Strangely, our observations favor a flat Universe. This means that we simply don’t see a large enough portion of the Universe to choose one option over another. So the Universe could very well have the shape of a cylinder or a doughnut—we just don’t know.
45. Are there states of matter in space other than liquid, solid, and gas?
In our Galaxy, at least three other states of matter are encountered. They depend on the space occupied by electrons orbiting around the nucleus of each atom. The first of these states is called plasma. It consists of atoms from which all or part of the electrons are detached. The remaining part of the atom is called an ion and carries a positive charge. The mixture of ions and electrons in plasma easily carries electrical and magnetic fields, and this is the state in which the substance that makes up stars exists. The other two states of matter are called degenerate. They arise in dead stars compressed by gravity. Electron-degenerate matter is encountered in white dwarfs, where electrons are squeezed very close to atomic nuclei. Such matter can be incredibly dense but still has gas-like properties. The radius of white dwarfs increases with decreasing mass. Baryonic degenerate matter is present in neutron stars. Here, electrons are pressed into nuclei, where they merge with protons, producing neutrons.
46. How do we know if the Universe is infinite or just very large?
The simple answer is that we don’t know for sure whether the Universe is truly infinite or just so vast that it’s beyond our comprehension. If it’s infinite, we could fly forever in a chosen direction and never come back. But if it’s not infinite, then embarking on such a journey might eventually lead us back to our starting point because the Universe is curved and closed upon itself.
Imagine Earth. We circumnavigate it and return to the starting point. The same could be true for the Universe. In such a closed Universe, a beam of light would travel in circles. This means that we might see an object by looking in the opposite direction, as our lines of sight would eventually converge.
Astronomers have analyzed images of very distant regions of the Universe looking for familiar patterns. If they found repeating patterns, it would be possible to calculate its size. However, no repetitions have been found, indicating that we live in either an incredibly vast or infinite Universe.
Even if it is infinite, we cannot see it in its entirety because the speed of light is not infinite. Even at the speed of 300,000,000 meters per second, it would take billions of years to travel across the Universe. So, any path of light beams is limited by the age of our Universe, which is approximately 13.7 billion years old.
47. What celestial body appeared first?
In the early Universe, either stars or black holes emerged first. In the former case, these were likely supermassive stars, possibly thousands of times more massive than the Sun, attracting each other and forming the first galaxies. If black holes appeared first, they could have served as the seeds for the earliest galaxies. Some of the material being drawn in fell into orbit around the black holes, beginning to form the stars that populate galaxies. Next-generation ground-based and space telescopes may attempt to discern the earliest celestial objects in extremely distant regions of the Universe.
48. How far can we see into space with the naked eye?
Without telescopes, we can see neighboring galaxies, such as the Andromeda Galaxy, the nearest large spiral galaxy to us, located about 2.5 million light-years away. The light coming from there started its journey before humans appeared on Earth. While this may seem far, telescopes have detected galaxies at distances of 12 billion light-years or more. Some gamma-ray bursts are visible to the naked eye from such distances, but these are brief events.
49. What is antimatter?
Antimatter is matter composed of antiparticles. Each antiparticle is a kind of mirror image of a corresponding elementary particle, possessing the same mass but with opposite signs for certain characteristics (such as electric and “color” charges, baryon and lepton quantum numbers).
There is a problem known as the baryon asymmetry of the Universe. It’s unclear why our world is almost entirely made of ordinary matter and where all the antimatter that should have been created when the Universe began has gone. Probably, ordinary matter had some advantage, leading to the survival of atoms after annihilation (when matter and antimatter collide, they mutually annihilate, producing photons and other particles different from the initial ones).
The Alpha Magnetic Spectrometer (AMS) particle detector aboard the International Space Station is monitoring antimatter. If it detects whole antiatoms, it would confirm the existence of antigalaxies, anti-stars, anti-planets, and possibly life made of antimatter somewhere.
50. What is anti-gravity, and how can it be achieved?
Anti-gravity refers to a force that repels objects rather than attracting them to each other, where the repulsive force is determined by the masses of each object and the distance between them, similar to how gravity operates. The exclusively attractive nature of gravity has always been a mystery, and we have not yet approached an answer to it.
The discovery of accelerated expansion of the Universe may indicate the existence of an anti-gravitational force stretching space, but no one knows what it entails. If we could detect anti-gravity, it would open up possibilities for realizing many fantastic ideas, such as anti-gravity propulsion, force fields, and even time travel.
