# How can space or the universe be both infinite and expanding at the same time?

By using the word “expanding,” we’re given the assumption that the universe must be expanding into something, from a central, fixed point. In order to explain it in a somewhat concise manner, you have to realize that space isn’t something that is just there and stellar objects are just moving throughout it, space bends and stretches.

An accurate analogy would be the dough/raisin analogy that’s commonly used: the universe is a blob of dough with raisins spread throughout it. When you put the dough into an oven, the dough expands, taking the “stationary” raisins with it, increasing the distance (space) between the raisins.

Without any way to leave our little raisin, there is no way to see the “end” of the dough – so, if we assume that the universe is truly infinite, then it simply doesn’t have anything to “expand” into. Using a mathematical analogy, you have a series of whole integers that start with one to infinity. If you took every number and multiplied by any factor, the numbers in your series have stretched away from each other; yet the list hasn’t expanded. You still have a series that progresses from a number to infinity.

# How do we know that fusion reaction takes place on the Sun?

Hydrogen is the most common element in the Universe, followed by Helium. When nebulae condense to form stars, there is immense pressure and heat generated. These conditions enable the Hydrogen atoms to fuse in a series of steps to form Helium.
The proof lies in the by products formed by the reaction, like neutrinos, positrons and gamma rays. These have been detected by scientists and agree with the established equations for nuclear fusion.
Further proof is that we can detect Helium by analysing the electromagnetic spectrum of the radiation from the Sun.
Helium was first discovered in the Sun and is named after the Greek Sun god, Helios.

# Why can’t you look directly at an eclipse?

An eclipse is just the sun being partially concealed by the moon. So the parts of the sun that you can see are as bright as they are at normal times. But because so much of the sun is shaded, less total light reaches you, so you don’t have the same reactions, like squinting turning away and restricting the size of your pupil, to let less light in and protect your eye.

So the brightness of the sun’s surface is being focused onto the back of your eye, for a long time, at more than the usual brightness. This causes damage.

When the bright surface of the sun is fully covered, then it is safe to admire the spectacular sight of the sun’s outer corona. But make sure to put proper solar glasses back on as soon as any of the sun becomes visible. Yeah, you really shouldn’t look at the sun.

# How was the Moon formed?

Even though it is lit up by thousands of stars, the night sky would seem vacant to most of us without the presence of Moon. The only natural satellite of Earth, the Moon has been the subject of interest of man forever. Moon had a position of reverence in ancient civilizations and has a great stature in various mythologies.
Myths and stories aside, the cosmic body has been one of the favorites of scientific world. The scientists are more interested in Moon now than ever, even so far as researching about the possibility of living there. As long as they have been mulling over the formation of the universe and solar system and Earth, the scientists have been after Moon as well.
There are a number of theories regarding the origin of Moon. Each one has its merits and faults. One of the earliest modern theories about Moon’s formation was the Fission Theory proposed by George Darwin, son of Charles Darwin. He argued that Moon was a piece expelled from Earth while the latter was still in its rapidly spinning molten state. This theory was popular in late 1800s.
Other notable hypotheses are the Capture theory which states that the Moon was captured by Earth and the Accretion theory stating Earth and Moon were formed together. However, the most widely accepted theory of Moon’s formation is the Giant Impact Hypothesis. It was first put forward by Canadian professor Reginald Daly in 1940s. Later many scientists gathered evidence in support of this hypothesis.
According to GIH, the Moon originated as a result of collision between the Earth and a Mars-sized body named Theia billions of years ago. This collision produced a large amount of debris around the Earth, some of which accumulated to form the Moon. The Moon rotated about one tenth of the distance today, and gradually became tidally locked with Earth.
The Giant Impact Hypothesis has been able to explain some of the aspects of the Moon, like its angular momentum, the difference between the cores of two bodies. However, a number of conflicts have arisen challenging the Giant Hypothesis. One of them is the isotopic comparison between Earth and Moon. The lunar isotope samples show great similarities with Earth. It is against the belief that much of Moon’s mass came from Theia. Also the volatile elements in Moon are not being depleted as fast as they are supposed to be, considering the energy unleashed during the collision. More and more research is being conducted on the subject, and one day we might have the sufficient explanation.

You might also like:

# Why is the sky dark if there are billions of stars in the Universe?

 Billions of stars
The dark night sky paradox, often called Olbers’ paradox, named after Heinrich Wilhelm Olbers(a German physicist of the early 19th century), arises from the fact that the number of stars in the sky is assumed to be essentially infinite, and hence the night time sky should be ablaze with light, not shrouded in darkness.
If the logic were correct, no matter where we looked in the sky, the canopy of stars would be continuous, with no dark gaps between any two stars. The result would be a heavenly vault as dazzlingly bright and as hot as the Sun’s surface. A paradox arises from the seeming conflict between logic and the observed darkness of the night sky. The solution comes from the landmark cosmological discovery made by American astronomer Edwin Hubble. He demonstrated that the universe is expanding and the light from the receding galaxies is shifting towards the red end of the spectrum–just short of becoming invisible.
It follows from this that beyond a certain distance, any star or galaxy will be receding from us so rapidly that its light will be shifted out of the visible spectrum into the infrared or radio spectrum. The radiation we receive from celestial body is therefore no longer visible to our eyes. This would greatly reduce the number of stars and galaxies visible in the sky, and could explain nighttime darkness. Scientists agree that this red shifting must contribute to the darkness of the night sky.
There is another supportive explanation based on the fact that the speed of light always remains constant and it can not be boosted any further. One consequence of the fixed speed of light is that we can not observe any object farther than about 15 billion light-years away from Earth. Even if there are more galaxies beyond that distance, light from them would not have the time to reach us.

Related posts:

# What would happen if Earth passed through a comet’s tail?

Nothing, except that we would witness a breath-taking aerial fireworks display. The meteoritic dust in the comet’s tail would be burned up by the air’s friction, just as meteors are consumed and become what is commonly (and erroneously) known as shooting stars when they enter our atmosphere.

Moreover, while we tend to ridicule the ancients who considered comets omens of death and destruction, we should remember that the twentieth century had a same scare, too. On May 18, 1910, the head of the famed Halley’s Comet was scheduled to pass between the Earth and the Sun. Newspapers caused widespread apprehension by announcing that the tail contained a poisonous gas–cyanogens.

 Comet pills label

A worldwide scare ensued and quite a few quacks has a field day. They encashed on the panic by selling comet pills, which according to their phony claim, would safeguard people from the ill-effects of the supposedly toxic gas. The long-awaited day arrived but nothing happened. In fact, no one saw the head of the comet against the Sun, although some reported the comet’s tail was faintly visible. The Earth had passed through the tail of Halley’s Comet, with all living beings remaining as safe and sound as they were before.

Comet tail (Wikipedia)

Related posts:

# What is ‘Grandfather Paradox’ concerning time travel and how can it be resolved?

The paradox manifests itself when you stretch Einstein’s general theory of relativity to its extreme limit. The theory, in principle, allows you to go back in time and visit the past. All you need to do is to take a trip through what is called wormhole, a theoretical tunnel that connects one part of the universe with another. You would emerge in a different space-time, having accomplished time travel to an event in your own past.

Consider our nearest galactic star Alpha Centauri which is 4.2 light-years away. A light beam takes that much time to reach it when traveling through ordinary space, but you could beat a light beam by taking a shortcut through a wormhole. (See diagram.) You start your journey in the year 2012 and emerge at Alpha Centauri in 1912 instead. Now for the paradox that defies logic: What if you travel much further backwards and accidentally kill your grandfather, thereby preventing yourself from ever being born on the first place? If you were never born, you could never go back in time, and so you could not kill your grandfather.

This paradox is so potent that scientists are unable to offer any rational solution. However, science fiction writers, having more inventive minds and greater freedom of imagination, have a way of circumventing such conundrums. They say that after traveling through the wormhole you emerge in a new universe where you can not encounter your grandfather — so never have a chance to gun him down. Period.

Below is a video explaining the paradox. Note that HBA is not affiliated with creator(s) of this video.

Related post:

# How do scientists know that dark energy is speeding up the expansion of the universe?

One of the great conundrums in astronomy is the nature of something called dark energy, a kind of anti-gravitational force that appears to be pushing the accelerator pedal of the universe. It has been known since long that the universe was born about 13.7 billion years ago and has been expanding ever since. Yet until quite recently scientists thought that the rate of expansion was slowing down. In the mid-1990s, they found to their amazement that they were utterly wrong. The universe was speeding up and still continues to expand at an accelerated rate.

The discovery was made quite accidentally. Two teams of astronomers were measuring the brightness of very distinct Type 1a supernovae. These stellar explosions are among the brightest events in the universe, which makes them an ideal tool for determining their distances. The light from a Type 1a supernova follows a predictable path, always peaking at the same level of brightness. The light itself has a specific attribute. Its intensity varies inversely (oppositely) with the square of the distance between the source such as a supernova and the surface such as a telescope’s mirror.

In other words, if the distance increases, the illumination decreases by the square of the distance. To cite an example, if a surface that receives 1 lux of light at a distance of 1 meter from the source is moved 2 meters from the source, that surface will receive 1/4 lux of light. Astronomers, who were using supernovae as ‘standard candles’ discovered that every remote Type 1a supernovae were significantly fainter than they should have been, based on their distances estimated using other techniques. This meant that these supernovae were farther away than originally thought, so the implication was that the rate of expansion of the universe must be accelerating.

We don’t know that exactly is the repulsive force that is causing the universe to expand, so it has been called dark energy, and yet astronomers are totally in the dark about its physical nature.

Supernova (Wikipedia)
Dark energy (Wikipedia)
Accelerating universe (Wikipedia)
Metric expansion of space (Wikipedia

Related posts:

# What is the chemical composition of the Sun? How do astronomers identify the types of elements it contains?

 Sun
The flaming gaseous mass of the Sun contains many of the chemical elements present on the Earth. When a ray of sunlight is analyzed by a spectroscope and split up into its colors to form a spectrum, the bands of color seen denote the elements present in the Sun. Each chemical element when heated to incandescence emits light of a particular wavelength, producing a characteristic color in the spectrum. Though some wavelengths are absorbed by the gasses in the Sun’s envelope or in the atmosphere, they nevertheless leave dark absorption lines in the spectrum, and their presence can be discovered.
The Sun, like most other stars, is made up mostly of atoms of the chemical element hydrogen. The second most plentiful element in the Sun is helium, and almost all the remaining matter consists of atoms of seven other elements. For every 1 million atoms of hydrogen in the entire Sun, there are 98,000 atoms of helium, 850 atoms of oxygen, 360 atoms of carbon, 120 of neon, 110 of nitrogen, 40 of magnesium, 35 of iron and 35 atoms of silicon. So, about 94 percent of the atoms are hydrogen.