What holds the galaxies together? Why is the universe still expanding from the Big Bang, and will it ever collapse again in a big crunch? Cosmologists-scientists who study the large-scale structure of the universe-ask these kinds of questions. Here are some answers they’ve come up with and some areas where they’re still in the dark.

The mysteries of gravity

Gravity is everywhere. It keeps our feet planted firmly on the ground, it holds the planets in orbit around the Sun and it keeps the stars and galaxies from flying apart. Basically, gravity holds the universe together. It’s also the most mysterious of nature’s forces.

The expanding universe

In 1929, astronomer Edwin Hubble discovered that other galaxies are racing away from our galaxy, the Milky Way. In fact, the farther away the galaxy is, the faster it’s receding.

Dark matter and the fate of the universe

Only 10 per cent of the universe radiates light, the other 90 per cent is invisible to us. This "dark matter" is vital to our understanding of the fate of the universe Read More
What holds the galaxies together? Why is the universe still expanding from the Big Bang, and will it ever collapse again in a big crunch? Cosmologists-scientists who study the large-scale structure of the universe-ask these kinds of questions. Here are some answers they’ve come up with and some areas where they’re still in the dark.

The mysteries of gravity

Gravity is everywhere. It keeps our feet planted firmly on the ground, it holds the planets in orbit around the Sun and it keeps the stars and galaxies from flying apart. Basically, gravity holds the universe together. It’s also the most mysterious of nature’s forces.

The expanding universe

In 1929, astronomer Edwin Hubble discovered that other galaxies are racing away from our galaxy, the Milky Way. In fact, the farther away the galaxy is, the faster it’s receding.

Dark matter and the fate of the universe

Only 10 per cent of the universe radiates light, the other 90 per cent is invisible to us. This "dark matter" is vital to our understanding of the fate of the universe. Without enough dark matter to put on the "gravitational brakes," cosmic expansion will continue thinning out the universe forever.

© Canadian Heritage Information Network, 2003

Black hole

Artist's conception of a black hole.

NASA / Dana Berry (STScI)

© NASA / Dana Berry (STScI)


Gravity is what holds the universe together. It keeps the galaxies from flying apart, the planets in orbit around the Sun, and our feet firmly on the ground. But we didn’t always know what gravity is. To some extent, it’s still a mystery.

The world according to Newton

In 1687, Sir Isaac Newton proved that gravity is the mutual force of attraction between all masses in the universe and that it extends infinitely across space but gets weaker with distance.

For us, the most massive object around is the planet we live on. The Earth’s mass is about 6 trillion trillion kilograms-that’s ""6"" followed by 24 zeros. That much mass puts out a lot of gravitational force, and all that gravity pulls down towards the centre of the Earth. That’s why no matter where you live on the Earth, you can’t fall off the "bottom"-gravity always pulls you towards the Earth’s centre.

Gravity also pulls the Moon toward the Earth. But because of its orbital motion, the Moon doesn’t hit the Earth, it basically falls around it. Likewise, Newton concluded that the Sun’s gravity pulls Read More
Gravity is what holds the universe together. It keeps the galaxies from flying apart, the planets in orbit around the Sun, and our feet firmly on the ground. But we didn’t always know what gravity is. To some extent, it’s still a mystery.

The world according to Newton

In 1687, Sir Isaac Newton proved that gravity is the mutual force of attraction between all masses in the universe and that it extends infinitely across space but gets weaker with distance.

For us, the most massive object around is the planet we live on. The Earth’s mass is about 6 trillion trillion kilograms-that’s ""6"" followed by 24 zeros. That much mass puts out a lot of gravitational force, and all that gravity pulls down towards the centre of the Earth. That’s why no matter where you live on the Earth, you can’t fall off the "bottom"-gravity always pulls you towards the Earth’s centre.

Gravity also pulls the Moon toward the Earth. But because of its orbital motion, the Moon doesn’t hit the Earth, it basically falls around it. Likewise, Newton concluded that the Sun’s gravity pulls the Earth toward the center of the solar system and keeps all the planets in orbit.

Einstein’s general theory of relativity

About two centuries later, in 1915, Albert Einstein predicted how gravity would act near massive objects like stars, which have very strong gravity. He suggested that we think of space as a flat rubber sheet that is stretched tight. A large ball placed on this sheet will make a dent. When a smaller object is placed nearby, it rolls into the dent and "gravitates" toward the larger object. The more massive the object is, the deeper the dent, and the greater the gravitational attraction.

A black hole is an extremely massive object embedded in the sheet. It makes such a deep dent that even light can’t escape.

© Canadian Heritage Information Network, 2003

Earth and Moon

Gravity holds the Moon in orbit around the Earth.

NASA / U.S. Geological Survey

© NASA / U.S. Geological Survey


See how gravity warps space.

Since Einstein, we know that matter distorts the geometry of the space around it. For example, a ray of light passing near a large object is deflected by it.

Canadian Heritage Information Network

© Canadian Heritage Information Network, 2003


Gravity actually determines what the "weight" of an object is. Every object, including you, has a certain mass. If you move that object to a planet with a different gravity, it will "weigh" a different amount because a different amount of gravity will be pulling on it.

If you went to the Moon, you’d weigh less because the Moon only has about one-sixth the gravity of Earth. So if you weighed 60 kilograms on Earth, you’d only weigh 10 kilograms on the Moon. But on Jupiter, which is more massive than the Earth, you would weigh much more-130 kilograms. And in space, far from any celestial objects, you’d weigh practically nothing.
Gravity actually determines what the "weight" of an object is. Every object, including you, has a certain mass. If you move that object to a planet with a different gravity, it will "weigh" a different amount because a different amount of gravity will be pulling on it.

If you went to the Moon, you’d weigh less because the Moon only has about one-sixth the gravity of Earth. So if you weighed 60 kilograms on Earth, you’d only weigh 10 kilograms on the Moon. But on Jupiter, which is more massive than the Earth, you would weigh much more-130 kilograms. And in space, far from any celestial objects, you’d weigh practically nothing.

© Canadian Heritage Information Network, 2003

Jupiter's Gravity

Jupiter's gravity is much stronger than Earth's. If you weighed 60 kilograms on Earth, you'd weigh 130 kilograms on Jupiter.

NASA / CICLOPS / University of Arizona

© NASA / CICLOPS / University of Arizona


In 1924, the American astronomer Edwin Hubble began studying spiral nebulae. Back then, astronomers thought spiral nebulae were cloud-like eddies within the Milky Way. They didn’t know there were other galaxies in the universe, and the Milky Way was thought to be a simple stream of stars in an otherwise vacant cosmos.

Hubble found that there were stars in the "Andromeda nebula." By carefully measuring their brightness he concluded that the nebula was really a huge island of stars far beyond the Milky Way. The nature of spiral nebulae was finally resolved and the Andromeda nebula was re-classified as a galaxy.

By 1929, observations of other spiral nebulae led Hubble to conclude that the Milky Way was surrounded by galaxies, and that these galaxies were all moving away from us. In fact, the farther away the galaxies were, the faster they were receding.

Growth rate

Knowing the rate of expansion is key to understanding the age, size and destiny of the universe. Astronomers are constantly seeking to refine the Hubble constant, a ratio of speed to distance that measures the expansion rate.

Read More
In 1924, the American astronomer Edwin Hubble began studying spiral nebulae. Back then, astronomers thought spiral nebulae were cloud-like eddies within the Milky Way. They didn’t know there were other galaxies in the universe, and the Milky Way was thought to be a simple stream of stars in an otherwise vacant cosmos.

Hubble found that there were stars in the "Andromeda nebula." By carefully measuring their brightness he concluded that the nebula was really a huge island of stars far beyond the Milky Way. The nature of spiral nebulae was finally resolved and the Andromeda nebula was re-classified as a galaxy.

By 1929, observations of other spiral nebulae led Hubble to conclude that the Milky Way was surrounded by galaxies, and that these galaxies were all moving away from us. In fact, the farther away the galaxies were, the faster they were receding.

Growth rate

Knowing the rate of expansion is key to understanding the age, size and destiny of the universe. Astronomers are constantly seeking to refine the Hubble constant, a ratio of speed to distance that measures the expansion rate.

Stretching universe

According to Einstein’s theory, the space between galaxies is what expands: the galaxies do not actually move through space. To picture this, imagine lampposts fixed to an expanding highway. As the road stretches, the space between lampposts increases.

© Canadian Heritage Information Network, 2003

See the expansion of the universe.

It assumes that at the beginning, there was already empty space in which an explosion could occur. In the expanding universe model, the assumption is rather that the fabric of space itself is expanding. The galaxies are not drifting farther apart from one another in space, but rather space itself is carrying them along as it expands, as in the example of the lamp posts along a highway that is stretching out.

Canadian Heritage Information Network

© Canadian Heritage Information Network, 2003


Expanding Highway

Imagine this highway being stretched like a rubber band to double its length in only a second. Two lamp posts next to one another will therefore be 10 metres farther apart after a second. In the same time period of a single second, two lamp posts that were initially 50 metres apart will be an additional 50 metres apart. The speed at which they move away from one another will be five times faster than the two adjacent lamp posts.

Canadian Heritage Information Network

© Canadian Heritage Information Network, 2003


We can detect dark matter by the effect that its gravity has on the visible universe. Galaxy clusters move as if they contain more mass than we can actually see. Computer studies show that galaxies are imbedded in vast halos that contain 10 times more mass than we can see-this mass is dark matter.

So what is dark matter made of? One possibility is MACHOs (Massive Compact Halo Objects). MACHOs are slow-moving chunks of cosmic material, perhaps burnt out stars, or Jupiter-sized objects, swarming in galactic halos. Or it could come from WIMPs (Weakly Interacting Massive Particles), which are fast-moving, exotic particles, left over from an earlier cosmic epoch.

So far, only a few MACHOs have been detected, but no exotic WIMPs. However, neutrinos now seem to be a promising dark matter candidate. Neutrinos are subatomic particles with no electrical charge and very little mass. They are so numerous that even with one-fifty thousandth the mass of an electron, they could account for as much matter as the entire visible universe.

Why does it matter to us?

Dark matter does matter. In fact, it could tell us whether the universe will end in a bi Read More
We can detect dark matter by the effect that its gravity has on the visible universe. Galaxy clusters move as if they contain more mass than we can actually see. Computer studies show that galaxies are imbedded in vast halos that contain 10 times more mass than we can see-this mass is dark matter.

So what is dark matter made of? One possibility is MACHOs (Massive Compact Halo Objects). MACHOs are slow-moving chunks of cosmic material, perhaps burnt out stars, or Jupiter-sized objects, swarming in galactic halos. Or it could come from WIMPs (Weakly Interacting Massive Particles), which are fast-moving, exotic particles, left over from an earlier cosmic epoch.

So far, only a few MACHOs have been detected, but no exotic WIMPs. However, neutrinos now seem to be a promising dark matter candidate. Neutrinos are subatomic particles with no electrical charge and very little mass. They are so numerous that even with one-fifty thousandth the mass of an electron, they could account for as much matter as the entire visible universe.

Why does it matter to us?

Dark matter does matter. In fact, it could tell us whether the universe will end in a big crunch or keep expanding forever. Depending how much mass exists in the universe, there are three possible scenarios:

-If the universe has more than enough mass, gravity will eventually stop the expansion of the universe, and everything will fall back together in a ""big crunch."" This is called a ""closed universe.""
-If the universe has just enough mass, it will keep expanding forever, but at a slower and slower rate-it will never stop. This is called a ""flat universe.""
-If there is less than enough mass, the universe will expand forever, but at an accelerating rate. This is called an ""open universe.""

So far, it looks like number 3 is what will happen. Even with the mass from MACHOs, WIMPs and neutrinos, there isn’t enough mass to stop the universe from expanding forever.

© Canadian Heritage Information Network, 2003

Galaxy cluster Abell 2218

In this image of galaxy cluster Abell 2218, the light from objects behind the cluster is spread in arc-like patterns by the cluster's gravitational field. This is similar to what happens when light passes through a glass ball. However, the mass needed to produce this distortion is much greater than the mass we can actually see. Images like these confirm the existence of dark matter.

W.Couch, University of New South Wales R. Ellis, Cambridge University

© NASA/STScI


Canada's Sudbury Nutrino Observatory

Canada's Sudbury Neutrino Observatory (SNO) is located 2000 meters underground in a nickel mine near Sudbury, Ontario. SNO detects neutrinos from the Sun and other astrophysical objects.

E.O. Lawrence Berkley National Lab

© E.O. Lawrence Berkley National Lab


Big Bang and Big Crunch

If the density of the universe is equal to a specified critical value, expansion will gradually slow down until it stops completely at the end of an infinite time period. This is the flat universe model. On the other hand, if the density of the universe exceeds the critical value, gravitational force will stop the expansion. When expansion stops, the universe will collapse into itself, and return to its initial state in a form of reverse Big Bang, called the Big Crunch. This is the closed universe model.

Canadian Heritage Information Network

© Canadian Heritage Information Network, 2003


Learning Objectives

The learner will:

  • Develop enthusiasm and continuing interest in the study of science
  • Appreciate some of the questions and research associated with astronomy
  • Understand gravity, Einstein’s general theory of relativity, and how it relates to space
  • Describe black holes, dark matter, open universe, and closed universe
  • Discuss scientific predictions about the future of the universe

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