The universe is constantly expanding, and the farther away you look, the farther away you look back in time. This fact is very important in understanding the universe as a whole.
Everything we observe around us is made up of atoms. Atoms are very dense particles, and they are made up of even more dense particles: electrons, neutrons, and protons.
As the universe expands, the density of atoms decreases because there is more space between them. This means that it takes longer for atom to interact with other atom.
When we look at distant galaxies, we are actually looking back in time. We are seeing how they looked many years ago when the universe was much smaller. Because of this, most galaxies appear to be moving away from our own galaxy, the Milky Way.
This expansion explains why galaxies move away from us
As we’ve discussed, the Big Bang was the event that created our universe. Following the Big Bang, everything in the universe was crammed into an incredibly small point – a point so small it could be considered nothing.
As the universe expanded, this point became filled with matter and energy. This is known as a cosmological principle – it states that there is no distinction between whether something exists within our universe or outside of it. There is only one Universe!
Over time, this expanding matter and energy increased in volume, creating pockets of gravitational pull that drew other matter and energy into it. As these new masses came into existence, they began to move away from each other, creating what we refer to as “space”.
The reason we can’t see any of this space is because it does not have any substance or energy within it – just darkness. Although we can’t see this space, its presence affects the way we view other things in the Universe.
The cosmic microwave background
In addition to observing the velocity of galaxies relative to our own Milky Way galaxy, astronomers also study the velocity of galaxies relative to each other and to the universe as a whole.
This is made possible by something called the cosmic microwave background (CMB) radiation.
The CMB is a fundamental electromagnetic radiation field that was present at the moment our universe began via the Big Bang. It has since cooled down and occupies every nook and cranny of space.
It is analogous to how water cooling down will result in a constant temperature throughout all containers, including outer space. The water cools down to its fundamental temperature, which in this analogy is zero degrees Celsius (or Kelvin).
Like cooling water, the CMB has cooled down to a uniform temperature of 2.7 Kelvin (approximately -270 degrees Celsius). This allows scientists to derive what its original temperature was, and therefore what occurred during the early stages of our universe.
Dark energy
A significant component of the Universe is something called dark energy. Dark energy is a theoretical property of space that causes the Universe to expand at an increasing rate.
We refer to this as energy, but it is not the same kind of energy we experience in our daily lives. Dark energy doesn’t seem to relate to us in the ways we experience other forms of energy, such as heat or electricity.
Rather, it appears to be some kind of scalar field, one that may have different values in different regions of space. We don’t know much about this field, except that it has a negative pressure — so in some ways it resembles a vacuum.
Because the Universe is expanding at an ever-increasing rate, this means everything else is moving away from each other. Because we perceive time as happening in a linear fashion, it looks like everything is moving forward into the future. But from the perspective of the overall expansion of the Universe, everything else is actually moving away from us.
Dark matter
Dark matter is a mysterious substance that nobody has ever seen or touched, but which scientists believe exists based on how mass in the universe is understood.
Based on the theory of relativity, it is thought that the mass in the universe is made up of 5 components:
baryonic matter (normal matter such as atoms and neutron stars), vacuum (space itself), dark matter, dark energy, and lastly, photons (the particle part of light).
Since baryonic matter and photons are observed to move at the same speed as each other, it is assumed that all things that have observable mass also have some form of gravitational force. This pulls other objects towards it.
Therefore, if you were to place something with no observable mass on a flat surface made of material with no gravitational force, it would not move.
Our galaxy is moving too
All of the billions of galaxies in our universe appear to be moving away from each other, as we observe them now, in the present day.
This was not always the case, however. Some galaxies were found to be approaching ours in previous observations.
How was this discovered? Astronomers use a tool called redshift to measure the motion of distant objects. It does this by measuring the wavelength of light coming from an object, and calculating how much that light has shifted toward the red end of the spectrum.
If an object is moving closer to us, then its light will have a shorter wavelength (or blue shift) than when it was observed earlier. If an object is moving away from us, then its light will have a longer wavelength (or red shift).
The reason for this has to do with physics-when an object is moving, its photons experience more force as they travel through space. This causes their wave length to increase.
Our solar system is moving too
As early as the 19th century, scientists had a pretty good idea that the Earth was moving.
We know this because we’re able to detect that our planet is moving relative to other things. For example, we know the Earth is rotating because we can see different parts of the world passing by each other as we move from one side of the planet to the other.
We also know that we’re moving in relation to observable objects in space, like other planets and stars. This is known as our velocity, and it’s measured in meters per second (m/s) or kilometers per hour (km/h).
Stars and planets move away from each other over time
All matter in the universe is moving, as a result of the expansion of the universe that was mentioned earlier. All matter is being pushed away from other matter by gravitational force.
As mentioned above, this phenomenon can be observed on our planet. However, since we live on a flat surface, we can’t see how things are moving away from each other in all directions.
We can only observe this phenomenon in space where there is no influence from a flat surface. Because of this reason, astronomers use observations made from space to determine the rate at which things are moving away from each other in the universe.
It was recently found that virtually all galaxies in the observable universe are moving away from our own galaxy – the Milky Way. This was determined by observing billions of distant galaxies and analyzing their spectral signatures – or how their light waves interact with one another.
Objects move away from each other because of gravitational attraction
We live in a universe that’s vast, complex, and — as you may have heard — expanding.
The space between objects in the universe isn’t truly empty, but instead is filled with a substance known as vacuum. This medium consists of subatomic particles such as gluons and photons, which are responsible for the force that holds atoms together.
Because gravity is a force that pulls objects together, it has the opposite effect when it comes to the expansion of vacuum. Because of this, scientists believe that the Universe is expanding even at the most distant corners.
But why do virtually all the galaxies in the universe appear to be moving away from our own? After all, if everything is expanding, wouldn’t we expect everything to be coming closer together?
As it turns out, there’s another phenomenon at work: rotational motion. When we observe our surroundings from our vantage point on Earth or in orbit around the Earth, we see that everything is spinning.
