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[♪ INTRO]

Humans have been looking up at the stars for thousands of years, but it was only within

the past century that we learned there's more than one galaxy out there.

Then, ever since the 1920s, astronomers have been finding more and more,

farther and farther away.

But how many galaxies are there in the universe, really?

Well, we'll never have an exact number,

but astronomers have ways of coming up with pretty good estimates.

It doesn't seem like it'd be all that hard to calculate the total number of galaxies.

Just take a picture of the sky and count up all the ones you can see.

Right? Well, it's more complex than that.

How many galaxies your image will capture depends on how sensitive your equipment is,

and how long you leave it running to catch all the super dim stuff.

It also depends on what wavelengths of light it can detect.

Because the universe is expanding, light from distant galaxies gets stretched and shifted

toward the longer end of the electromagnetic spectrum.

This effect is known as redshift.

That means there could be galaxies whose light is redshifted so much that they're no longer

visible to your eyes or your camera.

So to see the oldest galaxies, with the most redshifted light,

you need special, infrared telescopes.

We've been using those for over a decade,

but we've been counting galaxies for much longer.

The first real step toward estimating the total number of them came back in 1996, when

NASA published the Hubble Deep Field Image.

Over a period of 10 days, the Hubble Space Telescope imaged a portion of the sky 1/30th

the diameter of the full Moon, using visible and ultraviolet light.

It was a portion you or I would say looked pretty darn empty.

But it definitely wasn't.

In fact, there were thousands of objects there, from pinpricks to large smudges.

Most were so faint that no telescope had ever seen them before.

Statistically speaking, the distribution of galaxies in the sky is uniform, so in each

speck of sky you have roughly the same number of galaxies.

Extrapolating from the Hubble Deep Field, that's 120 billion galaxies total.

A decade later, Hubble completed a new, deeper survey of a different part of the sky, producing

the Hubble Ultra Deep Field.

Thanks to the newer equipment, that upped the estimate to over 200 billion.

Altogether, with the various fields Hubble and other telescopes have provided, astronomers

estimated that there are between 100 and 200 billion galaxies in the observable universe.

And they've been working with that number for the past two decades.

But in 2016, a paper published in the Astrophysical Journal threw that number out the window.

It claimed a more accurate value was ten times that: 2 trillion galaxies.

To figure this out, the research team used deep space images from Hubble and other telescopes,

as well as work published by other astronomers about the very early universe.

See, nearby galaxies are relatively easy to count, because they're usually pretty bright.

But the farthest galaxies are mostly too dim to be detected.

In this study, the team found a way around that.

Because it takes light billions of years to travel from those galaxies to Earth, we can

only see them as they were in the distant past.

So by using data about the early universe, the scientists developed a new mathematical

model to estimate how many super far, dim galaxies are actually out there.

The model could infer the presence of those dim galaxies that couldn't be visually detected.

Even though they were too dim to show up in the image, the computer could tell they were

supposed to be there based on the number, mass,

brightness, and distance of the galaxies it could detect.

And that's where 2 trillion came from.

As they looked back in time and counted those distant galaxies, the team also noticed something

astronomers had seen before: The farther back they looked, the more galaxies there were.

In fact, when the universe was less than a billion years old,

there were way more galaxies than there are today.

That isn't actually that surprising.

We've thought for a while that, over time, early galaxies merged to make a smaller number

of physically larger galaxies.

This study just supported that.

But that also brings up a new problem: All those early galaxies don't actually exist anymore.

They likely merged billions of years ago, but because it's taken light so much time

to reach Earth, we can only see them from before those mergers happened.

So we don't really know how many galaxies there are in those parts of the universe.

This also means that there are galaxies so far away that their light will never reach

us, because the universe is also expanding.

They're beyond what astronomers call the cosmological horizon,

or outside the observable universe.

So the total number of galaxies in the entire universe is basically unknowable.

But that doesn't mean we won't stop learning,

and that 2 trillion number isn't necessarily our final count.

More powerful telescopes, like the James Webb Space Telescope, which will hopefully launch

in 2019, will allow astronomers to collect more accurate data.

The JWST is 100 times more powerful than the Hubble, and will be able to observe galaxies

in those longer wavelengths.

In the meantime, with at least 2 trillion galaxies out there, it's safe to say that

every point in the sky is completely covered by galaxies.

We definitely can't see all of them.

But it's kind of comforting to know they're out there, bathing us in invisible starlight.

Thanks for watching this episode of SciShow Space!

If you've ever wondered how many stars there are in the universe, we made a video about

that, too, and you can watch it on the main SciShow channel.

[♪ OUTRO]

For more infomation >> How Many Galaxies Are There? - Duration: 5:35.

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Simple math proves how many ALIENS may be out there - Duration: 5:11.

Simple math proves how many ALIENS may be out there... a LOT more than you think!

Decades ago most people were afraid to admit that they believed in aliens that today things

have changed.

The universe is packed with 2 trillion galaxies, and anyone who believes that humans are alone

would be smug.

Recently a poll suggested that 54% of residents of the United States have confidence that

there are intelligent beings out there.

Some people might say that the optimism of those who believe is based on science fiction.

This is because no one has any concrete evidence of aliens.

However, many scientists agree that the cosmos is sprinkled with alien life and it may be

liberally sprinkled.

But is it possible to guess how close the closest alien beings are to humans?

Drake Equation Tells Us There Is Alien Life Out There

Back in 1961, an astronomer by the name of Frank Drake came up with an equation that

was simple for estimating the number of societies in the galaxy that are technically active.

This is known as the Drake equation, and it has been said to be the second famous, after

the E=mc2 of Einstein.

Anyone can look up this formula online, and they will see that it takes into account odds

there are planets that are habitable around other stars along with the chances that life

is going to arise and the probability that biology is going to evolve to produce beings

that are clever.

Without even delving into the Drake equation it is possible to use the reasoning that is

similar to work out how many alien societies there might be and how close to Earth they

are.

Recent research has revealed that one in six stars has a planet hospitable to life.

This is not one in a million, but one in six and this is the starting number that we can

take and go with.

Then some assumptions can be made and one, in particular, is if there were a million

worlds of Earth-size, what amount would ever have inhabitants are technically sophisticated?

Half Of All Planets Suitable For Life Could Have Produced Life

Life on Earth started quickly, a chemical activity that was random in 350 million trillion

gallons of ocean water spawned some molecules that could reproduce in a few hundred million

years.

With this in mind, biology does not need to be goaded too much to get started.

Therefore it is not unreasonable to work out that that around half of all planets suitable

for life are able to go ahead and produce it.

Of course, intelligence is far less certain.

Think about one in 100 biology encrusted planets having beings that can think.

Now according to Frank Drake assume that any alien life has continued to hang around for

around 10,000 years before then meeting an end.

If you do the math, it works out that about one in 100 million star systems could have

inhabitants that are technically adept.

This may be likened to the fraction of Powerball lottery tickets in the lottery.

So just how close are the closest alien lifeforms?

The average distance between the stars in our galaxy is 4.2 light years, which is the

distance to Proxima Centauri.

This means that for every cube of space with 4.2 light years on the side on average there

is one star.

Now consider a larger box that has 2,000 light years on the side, and this will have 100

million star boxes and perhaps a sophisticated civilization.

Closest Alien Life May Be One Or Two Thousand Light Years Away

This calculation is very rough and ready, but the closest alien lifeforms are perhaps

one or two thousand light years away.

This means that they are no closer than the Orion�s Belt three bright stars.

Of course, this does not mean that there aren�t aliens living farther or closer.

The order-of-magnitude indicates that they are not next door to Earth.

More than likely they do not have any incentive to pay a visit to Earth, and they may not

even know that Earth exists.

It is not possible for Earthlings to visit aliens on other planets either as even the

fastest of rockets would have to travel 20 million years just to reach other planets.

More than likely aliens do exist, and it is possible that 10,000 societies live in the

galaxy and most likely other galaxies.

However, while they will not be too close to Earth, they may be close enough to be discovered.

This is one of the main reasons why astronomers watch the sky for any radio signals that have

been launched.