Universe Today

How long ago did the Big Bang happen? What is the Big Bang's age? 13.7 billion years, plus or minus a hundred million years or so … that's the conclusion of a team of astronomers, working primarily with the latest WMAP data.

This estimate is of the age of the Big Bang itself, and is independent of other estimates of the age of the universe, such as the age of the oldest stars, and the oldest age of radioactive matter (one of the wonderful things about all these estimates is that they are consistent!)

How can you estimate the Big Bang's age simply by observing the cosmic microwave background (CMB)? Here's how …

Start with the microwaves that seem to come, uniformly, from all directions (of course there are all kinds of things which emit microwaves – dust, stars, galaxies, even free electrons spiraling around magnetic field lines – but they all have distinctive features, so they can be subtracted). These microwaves come from the electrons which filled the universe a long time ago, just before they combined with the protons to form neutral hydrogen atoms. At the time, the ordinary matter in the universe was a gas (actually a plasma), with a temperature that was everywhere the same (it was in thermal equilibrium), and emitted black-body radiation.

Once the plasma became a neutral gas, the thermal radiation streamed free … and a tiny, tiny part of it is observed today, by exquisitely sensitive instruments such as the Wilkinson Microwave Anisotropy Project (WMAP) ones. We see it as microwaves because it has been redshifted, due to the expansion of the universe.

The CMB is the most perfect blackbody known, but it's not a perfect blackbody! It has a hot and cold pole, 180^o apart, due to the motion of our solar system+galaxy. It also has fluctuations of about 1 to 10 µK (yes, that's micro-Kelvin, millionths of a degree), which look like random noise to our eyes (see above). But they're not (random); instead they are the imprints of various physical processes, mostly from the time the blackbody photons streamed free (some are due to effects of mass-energy along the path from primordial plasma to us). For example, sound; like any gas, sound could travel through the early universe's plasma, and the CMB photons retain information about the last sounds there (and then).

By carefully analyzing the fluctuations, the density of matter (and dark energy) at the time can be estimated, as well as composition of the matter (ordinary matter, neutrinos, dark matter). Plug those estimates into General Relativity equations and out pops an estimate of how fast the universe was expanding then. Compare that to an estimate of how fast the universe is expanding now (the Hubble constant), and you get an estimate of the age of the Big Bang. This WMAP page, and related ones, explains this in more detail; the main WMAP team paper on the Big Bang age is Five-Year Wilkinson Microwave Anisotropy Probe Observations: Cosmological Interpretation.

Other Universe Today articles on the Big Bang age include How Old is the Universe?, Planck Starts Collecting Light Left Over From Big Bang, and Beginning of the Universe.

And be sure to listen to the Astronomy Cast episode How Old is the Universe?.

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Rats! Another perplexing space mystery solved by science. New analysis of the famous "cold spot" in the cosmic microwave background reveals, and confirms, actually, that the spot is just an artifact of the statistical methods used to find it. That means there is no supervoid lurking in the CMB, and no parallel universe lying just beyond the edge of our own. What fun is that?
Read more…

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By combining the General Theory of Relativity and the Cosmological Principle, we are able to arrive at three possible space-time curvatures or geometries of the Universe: closed like a sphere, open like a saddle, and flat like a sheet of paper. The first geometry is equivalent to a finite universe while the other two are different versions of an infinite one.

A basic difference between the two, i.e., a finite and an infinite universe, can be arrived at when we imagine what would happen if a photon managed to travel on either one.

A photon that follows a straight path on a finite universe may end up in the same spot as when it first started out, while a photon that would head out in the same manner on an infinite one will never find its way back (as long as it doesn't turn back, of course!).

Is there a way to determine whether the Universe is infinite or finite? Apparently, its geometry (open, closed, or flat) is largely dependent on the density of matter in it.

That is, if the density of matter is larger than a certain value, known as the critical density, the geometry should be closed. If it is less than the said value, the geometry should be open. And if it is equal to the critical density, then it should be flat.

Measurements made on the cosmic microwave background radiation by WMAP point to a curvature that can be well considered flat. In other words, we most likely have an infinite universe.

Having an infinite universe, regardless of whether it is flat or open, somehow eliminates certain predictions as to how it will meet its own end.

Before WMAP's discovery, there have been 4 strong predictions on the ultimate fate of the Universe: a Big Crunch, a Big Bounce, a Big Freeze, and a Big Rip. Unless any solid evidence is found that will contradict WMAP's findings, we will have to do away with the Big Crunch and the Big Bounce models.

These two can only occur if there were possibilities of an eventual collapse. However, a collapse could only happen with a close or finite universe. In an infinite one, there should be a continuous expansion.

The main difference between the two different kinds of infinite universes, open and flat, is in the rate of expansion. In the former, the expansion will be forever and at a constant or even accelerated (by factoring in dark energy) rate. In the latter, the expansion will still be forever but at an asymptotically decelerating rate.

We have some articles in Universe Today that are related to the infinite universe. Here are two of them:

• Further Evidence Found for Dark Energy
• Dark Energy Dominated Universe

Infinite universe articles brought to you by Physics World, here are the links:

• The universe is flat – official
• WMAP gives thumbs-up to cosmological model

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

• Before the Big Bang
• The Big Bang and Cosmic Microwave Background

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A closed or finite universe is one in which, after having traveled in one direction on it, you can find yourself back to where you started.

Barring any new discoveries that may refute the WMAP's findings, it's official – the Universe is flat. That also means it should be infinite in extent. Still, that doesn't mean we can altogether throw other models, like that of a finite one, into the trash bin. After all, the colorful history of science has oftentimes been dotted by discoveries that have sea-sawed contradicting models.

Take the particle and wave theory of light for example. These two opposing theories strived to define the true nature of light. Eventually, it was found that both theories were correct after all. Of course, that doesn't happen all the time. But what if beyond the horizon the Universe is found to be in fact a finite one?

What then are the characteristics of a finite universe?

If our universe is finite in extent, that would only mean that it is closed or, for purposes of visualization, shaped like a sphere. That's the reason why we said earlier that if we traveled in one direction on it, we can find ourselves back at our point of origin.

A closed and finite universe may also mean that the average density of matter inside it is greater than what is known as the critical density. Combining what they know about the geometry and density of matter inside it, scientists can predict the possible ending our universe may have.

For one, when all matter has an average density greater than the pre-determined critical density, they (matter) would have enough gravitational force to pull the entire universe back into a singularity.

You see, ever since the Big Bang, the Universe's expansion has been influenced by a tug-of-war between its outward momentum and the gravitational force of all its constituents. If the gravitational force is large enough, the expansion will eventually reach a maximum and then a collapse will ensue.

Scientists who believe that a collapse is unavoidable are further divided into two groups: those who believe that the Universe will end in a Big Crunch and those who are more inclined towards a more peculiar end: a Big Bounce. Both of these endings can only be realized if one condition is also satisfied – that we have a finite universe.

We have some articles in Universe Today that are related to the finite universe. Here are two of them:

• Further Evidence Found for Dark Energy
• Dark Energy Dominated Universe

Infinite universe articles brought to you by Physics World, here are the links:

• The universe is flat – official
• WMAP gives thumbs-up to cosmological model

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

• Before the Big Bang
• The Big Bang and Cosmic Microwave Background

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Inflation Theory is an extension of the Big Bang model. It was created to answer certain questions that arose from observations inconsistent with or unexplained by the Big Bang. Basically, Inflation Theory talks about a period of very rapid expansion before a relatively gradual one.

The Big Bang Theory is consistent with many observations that we have today like the movement of galaxies away from each other (expansion), confirmed by red shifts, and the detection of the CMBR (cosmic microwave background radiation). However, there are other observations that don't fit into the puzzle.

Let me walk you through two of these observations as well as point out how Inflation Theory provides the necessary adjustments to make them fit into the Big Bang model.

First, measurements by the WMAP (Wilkinson Microwave Anisotropy Probe) show a universe geometry that doesn't coincide with more probable predictions of a Big Bang model. Supposedly, curvature would have increased as time progressed. WMAP measurements however reveal a geometry that's nearly flat.

By factoring in an extremely rapid expansion, we can understand how the Universe might have actually grown so large that it would appear flat from our point of view. It's like standing on the surface of the Earth. We know that the Earth is spherical but when we look all around us, it is as flat as far as our eyes can see.

Second, when we measure the distances between two of the farthest points located at opposite sides of the cosmos, they appear to be too far apart when compared with the distance that should be separating them. The expected separation is based on the current rate of expansion and the Big Bang model that suggests them to have been initially in contact.

That they could have actually been in contact is consistent with the near-uniformity of CMB temperature readings.

Once again, if we factor in inflation, and subsequently the idea that there must have been an exponential bust of expansion, the wide separations can be explained. Furthermore, inflation also explains how they could have been in contact and hence achieved the near-uniform temperature we're now detecting.

Scientists are still not sure whether the entity that caused inflation is the same entity that's driving accelerated expansions particularly in the outer regions of the Universe today. I'm referring to dark energy. Although it definitely looks almost like it, some scientists believe that whatever caused inflation only existed within that brief period of time and hence could not be dark energy.

We've got a few articles touching on the Inflation Theory here in Universe Today. Here are two of them:

• Cosmologists Search for Gravity Waves to Prove Inflation Theory
• Inflation Theory Takes a Little Kick in the Pants

NASA also has some more:

• What is the Inflation Theory?
• Inflation in the Universe

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

• Questions on Inflation
• Inflation

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There's a continuing discussion among astronomers regarding the actual shape of the Universe. Right now, the most widely accepted model supports that of a flat universe.

This has been confirmed through accurate measurements made by WMAP (Wilkinson Microwave Anisotropy Probe), a spacecraft that maps out the differences in temperature of the cosmic microwave background radiation (CMBR) across the entire sky. These results, which only has a 2% margin of error, were released in 2008.

Even before the WMAP measurements, a project in 2001 known as Boomerang already showed that the Universe was flat within 15% accuracy.

Before these seemingly conclusive findings were released, discussions revolved around three possible shapes: flat, closed, and open. To ascertain the actual shape, majority of astronomers were in agreement that they only needed to determine a few significant information about the Universe. One of them was its density.

They knew that if the density was found to be approximately equal to an accepted critical density, then the best prediction would be that of a flat universe. If it was lower than the accepted value, then the best prediction would favor an open one. Finally if it was higher than the critical density, the prediction would favor a closed universe.

For purposes of visualization, you could picture a sheet of paper for the flat shaped model, a saddle for the open, and a sphere for the closed.

All three models stem from the Big Bang cosmological theory, which strives to explain how the Universe began. This is easily the most acceptable of all cosmological theories, following the detection of the CMBR. It is only through the Big Bang theory that the presence of the CMBR can be explained.

Since the ultimate fate of the universe is also dependent upon its density, there is therefore a direct link between its shape and how it will end. The possible endings, which are also offshoots of the Big Bang, are: the Big Crunch, Big Bounce, Big Freeze, and Big Rip.

Observations have shown that the Universe is expanding at an accelerated rate, with the outermost regions moving farther away at velocities close to the speed of light; i.e., much much faster than those regions that are near us.

A flat universe would still favor an expansion but not at this rate. This observation has led scientists to predict the presence of a so-called dark energy that pushes the galaxies farther apart.

We have some related articles here that may interest you:

• Open Universe
• Density of the Universe

There's more about it at NASA. Here are a couple of sources there:

• Is the Universe Infinite?
• WMAP Facts

Here are two episodes at Astronomy Cast that you might want to check out as well:

• Questions on the Size, Shape and Centre of the Universe
• What is the Shape of the Universe

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How does one measure the size of the Universe? The challenge in measuring the size of the Universe doesn't lie on its enormity. Simply put, if you want to measure a really big area, all you need is a really long measuring stick.

In the case of the Universe, one of the means of measurement is provided by the cosmic microwave background radiation (CMBR), the living storyteller of the Big Bang. Now, there's no problem with picking that signal up. In fact, the Wilkinson Microwave Anisotropy Probe or WMAP has been able to generate a very detailed full sky map of the variations in temperature of the CMBR.

Information from WMAP has even led scientists to peg the age of the Universe at 13.7 billion years. This has tempted some people to believe that, since it took 13.7 billion years for the radiation to reach us then the edge of the Universe has to be 13.7 billion light-years away from us.

Remember that not only is the Universe expanding but that its rate of expansion is also increasing.

So this is where it gets a little tricky. Since the Universe is expanding in all directions and since in the regions very far from us, the galaxies are moving away at rates close to the speed of light, then signals coming from them may never reach us.

This is the reason why we're hearing terms like the 'observable universe'. There are simply regions out there that we cannot hope to peer into from where we stand. Furthermore, since we're not really sure how the rate of expansion has evolved since the Big Bang, we cannot ascertain where the source of the CMBR that we're picking up is now.

Imagine yourself and a companion playing catch – that is, your companion throws balls while you catch them. Assuming that your companion throws the balls at the same speed and that you know exactly how fast the balls travel, it would be easy to compute how far your companion is if you also knew the time of travel.

But that task would only be easy if you and your companion were staying in place. If any of you were moving (either away or toward one another), things would get a little bit more complicated. Much more if your rate of separation is not constant, i.e., with the ball in mid-air, you might separate slowly at first then speed up.

To end, what would happen if you would be moving away forever at practically the same speed as the ball? Hmmmm.

Some astronomers believe that the Universe is infinite, going on in all directions forever. Others have estimated that the Universe has expanded to the point that it's about 150 billion light years across since the Big Bang; and it's getting bigger every day.

We've got a few articles that relates to the size of the universe here in Universe Today. Here are two of them:

• Kepler Will Be Used to Measure the Size of the Universe
• Most Distant Object Ever Seen

NASA also has some more:

• Size of the Universe
• WMAP Facts

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

• How Big is the Universe?
• What is the Shape of the Universe?

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