# Astrophysics and Cosmology: Crash Course Physics #46

After a year of studying the laws of the universe
together, a year of studying and calculating, learning about motion and fluids, thermodynamics,
electricity and magnetism, light and sound, We find ourselves here: Space, the final frontier. We’re all voyagers on a mission to understand
the universe, using the power of physics. And even though we’ve been doing it for
centuries, there’s so much we have to learn
about the cosmos! Some of the most exciting research in physics today
is being done by astrophysicists and cosmologists. Astrophysicists study the physics of celestial
bodies, such as planets, stars, and galaxies. Their research takes us inside phenomena like
black holes and supernovae. But we can use physics to try to answer even
bigger questions about the universe. Cosmologists study the universe overall and
as well as its future. It’s their job, and yours, to continue looking
into the night sky, searching for answers using the
tools and knowledge that physics can provide. [Theme Music] Before we can talk about something as big
as the universe, we need to be able to describe
just how big it is. When talking about things on Earth, we typically
use measurements in the range of nanometers
to kilometers. But when we’re in space, we need something
a whole lot bigger. For instance, I could say that the nearest
star to earth, besides the sun, is 4×10^13
kilometers from us. But that’s a mouthful for something that’s
basically right next door, in cosmic terms. So it’s easier to say that this star, Proxima
Centauri, is 4.2 light-years from Earth. A light-year is a unit of length, with one
light-year equaling the distance that light would
travel in a vacuum in one year. If you take the speed of light, about 300
million meters per second, and multiply it
by how many seconds are in a year, you find that one light-year is approximately
10^16 meters, or 10 million, billion meters. To give you a sense of scale, it takes light just
over 8 minutes to travel from the sun to the Earth, and the Milky Way is roughly a hundred
thousand light-years in diameter. Sometimes we also use a unit called a parsec,
which is equal to 3.26 light years. Now, when we say that Proxima Centauri is
4.2 light-years away, this also means that when we look at the
star through a telescope, we’re seeing what
Proxima Centauri looked like 4.2 years ago. It takes light that long to get here from Proxima
Centauri, so we’ll never know what that star – or any star or other distant
object – looks like at this exact moment. This means that, as we observe celestial
objects far away, we’re looking into the past, seeing what stars and galaxies looked like millions,
if not billions, of years ago. While we’re observing these stars, we can use a
spectrometer, the device that separates wavelengths, to reveal the star’s absorption
spectrum and its elemental composition. But when we study very distant bodies, we find that their absorption spectrum is slightly different from what we’d expect, given our knowledge of typical star compositions. Remember the Doppler effect? How the pitch of an ambulance siren becomes higher
as it approaches you and lower as it moves away? The same effect happens with light! If an object is moving away from you, the
speed of light doesn’t change, but the peaks of the electromagnetic wave that it emits
move farther apart. This effect – which occurs with light emitted
by an object moving away from Earth – is called redshift, because the longer the
wavelengths get, the closer they are to the
red part of the visible spectrum. Once astronomers recognized and could
measure redshift, they found that the spectra from
nearly every distant galaxy was redshifted, meaning that every galaxy
was moving away from us. And if that wasn’t strange enough, astronomers such
as Edwin Hubble noted that the amount of redshift is
proportional to the distance from Earth. So the farthest galaxies are moving away even
faster than the close ones! Georges Lemaître, a Belgian physicist,
proposed this relationship as Hubble’s Law. It expresses the velocity at which a galaxy
is speeding away from Earth, in terms of its
distance from us. The equation uses a proportionality
constant called the Hubble parameter, which says that for every million light-years of
distance from us, a galaxy is moving away at an
additional 21 kilometers per second. Now, the fact is that nearby galaxies might
be going away from us or toward us, based on
how they’re moving within their local cluster. But the overall tendency for distant galaxies
to recede from us is much more common, so
Hubble’s Law holds true in most cases. And this trend of distant galaxies moving
away from us, and from one another, is called
cosmological redshift. By the way, this expansion looks the same
whether you’re on Earth or not. No matter where you are, all distant galaxies
appear to be moving away from you. So this leads physicists to believe that at
some point in time, all the stars and galaxies
were closer to one another – a lot closer. In the 1940s and 50s, Russian-American
physicist George Gamow, developed a theory of the
early universe that explained, among other things, why so many light elements,
like hydrogen and helium, were observed throughout
the cosmos. He suggested that the universe began in a
state of highly compressed hot plasma – a sort
of hot soup of elementary particles. His theory became known, somewhat dismissively
by his colleagues, as the Big Bang Theory. And this same theory ultimately predicted that
there should be radiation left over from the initial,
rapid expansion of that compressed plasma. This is because hot plasma, like the plasma
in the flame of a candle, is not transparent. But ordinary gas – like the air around a
candle’s flame – is transparent, and it lets
light travel freely through it. So it would make sense that the early, hot
universe was originally opaque, until it cooled
down to the point where it became transparent. Once that happened, the thinking goes, light
from the Big Bang was able to travel freely. But its wavelength kept stretching out, redshifting
until it could only be detected as microwave radiation. Gamow’s theory didn’t gain much acceptance,
and it was largely forgotten. Until, in 1964, American astronomers Arno Penzias
and Robert Wilson pointed a radio antenna into space,
and they discovered cosmic microwave background
the Big Bang, by accident. They found that a low-energy microwave radiation
persisted at all times, day and night, and they concluded that the source of the
support for the Big Bang Theory, and it tells us a lot about the conditions of
the early universe. Thanks to these insights, along with the observed
expansion of the universe and other evidence, we have learned that the universe began in
a hot dense state, then cooled, and produced
galaxies and clusters that we see today. However, many mysteries remain. For instance, if the universe started with
such high density and temperature, wouldn’t
gravity make its expansion slow down? The fact is, the rate of expansion would
slow down only if the universe was filled with
nothing but matter and radiation. But that’s not the case! Space is filled with a constant – or at least, slowly
varying – form of energy known as dark energy. And because of the pervasive presence of this
energy, according to general relativity, gravity is
actually causing space to expand, and accelerate! This isn’t just theoretical. Recent evidence suggests that the universe
actually IS accelerating in its expansion,
showing no signs of slowing down. But beyond the fact that it exists, there’s
not much that we know about dark energy. Another one of the universe’s great mysteries
is the existence of mass that we can’t see,
but we know that it, too, exists. When we study a galaxy’s rotation, we can
estimate how much mass is in it by measuring
its radius and rotational velocity. But when we actually calculate that mass,
the result is far greater than what’s observable
as stars and gas. The conclusion is that there’s an immense
amount of mass in the universe known as dark
matter, which doesn’t reflect or emit any light. By current estimates, dark matter makes up
almost 85 percent of all the matter in the universe. This means that all visible matter, including
stars and planets, make up just a small percentage
of all energy in the universe, while the rest is mysterious dark energy
and dark matter. Research in these fields is ongoing and
new evidence is found every year, refining our
understanding of the universe. In the past year here on Crash Course, and
for thousands of years, we have used physics to
answer some of life’s most important questions. Whether it’s a ball flying through the air
or the origin of the universe, we can use our knowledge from Newton’s Laws
to special relativity in order to move closer to
the truth. And there’s still so much to discover, both
in the stars and here on earth. Some of the most groundbreaking research is
happening on the smallest scales, as physicists seek to understand the building blocks
of our universe and the nature of matter itself. It’s not an easy task, and it’s why we
need scientists and enthusiastic supporters
such as yourself to go out, be curious, ask questions, and to find
looking in the distance is also looking into the past. We discussed redshift and used Hubble’s
Law to calculate how much certain parts of
the universe are expanding away from us. Finally, we introduced the Big Bang, cosmic
background radiation, and the mysteries of
dark energy and dark matter. Bye! Crash Course Physics is produced in association
with PBS Digital Studios. You can head over to their channel to check
out a playlist of their latest amazing shows like: The Art Assignment, Gross Science, and Deep
Look. This episode of Crash Course was filmed in
the Doctor Cheryl C. Kinney Crash Course Studio with the help of these amazing people and
our equally amazing graphics team, is Thought Cafe.

### 100 Replies to “Astrophysics and Cosmology: Crash Course Physics #46”

1. James Bjerno says:

I want to marry this girl

2. Me says:

Caught the animated Phil Plait in the beginning. His astronomy CC series is great!

3. sylvainr0 says:

You're probably great, but I can't stop thinking that a woman with a dark skin may have been put here for antiracist reasons, not for scientific reasons. Sorry, I will not look at you video.

4. Shivam Singh says:

I'm in FC

5. Science Stuff says:

I love this channel and Dr. Shini

i am confusion.

I wish i understood more than half the stuff she said

Someone else who needs to learn to take a breath between sentences. Lasted 2 minutes before I ran out of energy and went off to find a decent presenter.

9. Unofficial Memer says:

10. Brian Coley says:

I'm in love. I also love astronomy.

11. E114 says:

12. Sheldon Rozario says:

do asmr

13. Rushita Solanki says:

Excellent 🙏🙏🙏

14. kelso says:

why are the planets moving away from us so quickly? what did we do

15. EzAzAbc ForeverKing says:

I. WANT. MORE. MUST. LEARN. MORE. YOU PROVIDE ME NOW, PLEASE, SIR. BRAIN… HUNGRY. MORE SMALL PHYSICS HAVE MUST, PLEASE , SIR, MR.SIR, DUDE AND OR DUDET.

16. Sir Husny says:

Thankful. Thank you for increasing the love I have for astrophysics

17. Super Sara says:

What do you have to do to become a cosmologist?

at 0:52 is that Vsauce?

19. Larry stylinson says:

if galaxies are moving away from us does that mean that we are also moving away from them

20. Muskaan Taneja says:

I didn't get the concept at 5:12 … Anyone plzz tell me..how the opaqueness of space is related to candle flame

21. Maroof Ali says:

She is absolutely gorgeous…

22. Mindy Wu says:

why this feels like an ending of movie series like Harry Potter, where I felt sad and want more episodes.

23. Suren Nimalan says:

24. Vaibhav Tripathi says:

5:13 That's not plasma, in flame of a candle. Watch ted ED's new episode on weather flame is a liquid, solid or gas.

25. 塔诺Jahsoka says:

(Plz no one attack me)
The Bible said it first🙂 "God stretched out the heavens like a curtain"

26. mainraj fatima says:

What is the difference between Cosmologist and Theoretical Physicist?

how exactly is gravity accelerating the expansion of the universe?

When can a person study cosmology?
In master program or phd?

29. J D says:

@ 0:55. Hey…isnt that Phil Plait's animated guy?!?!

30. Isobel Joy says:

Could you maybe post the script somewhere please?

31. Justine says:

she’s so interesting to listen to

32. ThePhysicsLovers Community says:

And I've aimed becoming an astrophysicists right from my childhood

33. khushboo says:

Can an astrophysits become an astronaut.

34. Taylor Kohls says:

Any plans for a Modern Physics playlist anytime soon? I think Crash Course needs a series on that.

35. Legoracers says:

You can seek knowledge on Astronomy from the guidance of the Quran,by Dr Zaid Kasim Ghazzawi. The function of the black holes. Sirius star orbit of 49.9 years. 😊

36. Kháos Hooligan says:

Very interesting!!!

Hi I’m John Gre

Oops sorry. Wrong person

38. Krishna Anirudh says:

What is beyond space….?Will we be able to find it….Why ,the universe was created….who created that?….
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39. Ashianta M says:

sTAR TREK I—

40. Esther Baht Israel says:

Humans are destroying the world with these bombs

41. Boom Boom SHAKE THE ROOM SAY WHAT says:

she is so good

42. Haylazlo says:

♥️♥️♥️
I am so sad it is over 😭

43. Anil Khandge says:

I want a whole series on cosmology

am I the only one bothered on that unsolved rubiks cube?

45. Tash sofa says:

Wait! How can we see proxima centori 4.2 years ago if light travels for 8 minutes from the sun. Wouldn't we see proximal centori how it was 8 minutes ago?!

46. troll meh says:

i said it before, and ill say it again, you are my role model ! bless you :')

47. Sai Sasi says:

Hay , Really nice video with rich content.Good Work , Keep Up…!

48. VIsneW NaiR says:

Amazing presentation
i think she is from india🇮🇳😍

49. niko 155254 says:

After I think it deeply, I realize that crash physics is awesome. cuz CC always only teach the basics. and it only require me to watch an episode twice or thrice to make me understand the whole concept of the matter. while my teacher require 5 hour class to teacj me about inductivity

50. Top Hat Potato says:

what if the universe's expansion is actually the big bang not finished, like light taking a long time to reach long distances

51. ıllıllı Dᴇᴄɪʙᴇʟ ıllıllı says:

1:32 iN CoSmIc tErmS

52. horseofblack says:

at about 1:45 minutes she notes we see the light emitted from alpha centorie about 4 years ago. but then implies when you are looking at galaxies 14 billion light years out toward the edge of the visible universe, and 14 billion years younger than our galaxy, we know exactly what those galaxies are doing today, that today those galaxies are speeding away from us at an accelerated velocity. BOGUS interpretation of the data. do the math with time being negative.

53. Lorenx Anene says:

I LOVE YOU SHINI…I WATCHED ALL THE VIDEOS…THANKS A LOT

54. MuShroom Me says:

Ok

55. It's Ritu says:

I want this series to never end😭

56. Night Walker says:

why do fr. lemaitre not given enough credits for the big bang theory

57. Samisnotbritish says:

6:18 Our whole universe was in a HOT DENSE STATE

58. Tess Conroy says:

super cool. learned a lot . xie xie.

59. Ashley Lieber says:

I seem to recall the the Belgian Priest, Georges Lemaitre, proposed what is called the Big Bang Theory, not Gamov. Please let me know if this is incorrect.

60. Thomas Kellogg says:

1:06 "In 2020, CERN opens a cosmic portal that unleashes Cthulhu. Cthulhu runs for office and wins."

61. jp gpn says:

Just beautiful

62. Ben Dova says:

So is the universe stretching or is more and more matter being pulled out of somewhere to keep it expanding. Jw

63. VANDI PRANTHAN says:

I love astro physics that my dream is to become astrophysicist
That to meet aliens

64. Jason Medina says:

What if what we consider to be the "big bang" is just the other side of a black hole where all the matter already got sucked up and spit out and now its expanding because of the kinetic energy from being spit out on the other side of the immense gravitational pull. It never started from one initial point, instead it was simply "redirected"?

65. Piyush Kumar says:

Please make a series on cosmology

66. ICE GANG FERRARI F8 says:

great lesson but still more to learn I heard time remembering things, come find out why

67. little by little says:

"Do those who disbelieve not see that the heavens and the earth were (once) one mass all closed up, then We rent them apart. And it is from water that We created all life. Will they not believe"

Chapter 21 verse 30 Quran
1400 years ago a man named Mohamed (pbuh) told his people this, and he couldn't read or write !! so where did he get it from ?

68. Maruf Saika says:

Ssssssssssssssssssssssss

69. Alexis E. says:

I love cosmology 😊

70. lovely sen says:

i really want a whole series on quantum physics

71. Jonathan Davis says:

don't forget photons have to get past the inside of a star

72. TehDuckStalker's alt says:

poo in the loo

73. wolvenar says:

I miss this series and it's presenter.

74. кто-то с кем-то где-то там says:

👍👍👍👍👍🙌👌👌👌👌👌

75. dude decent says:

I'm afraid to die not because of dying but beacaus I will never see how far humanity will travel or what we will discover

76. mikeg689able says:

A new thought for common sense.

I think of gravity as this. Energy emitted from the sun instantly begins the rarefied process. Gravity is this process in reverse, energies negative byproduct. Through this frequency zones, if you will, are established.

By creation and use of the magnetosphere, a portion of the energy from the sun is centered at the earth's core. It is thereby emitted in the same fashion as the sun, at tolerable levels determined by earth.

If an individual were to think of this as a 1,2,3 rarefied process, the 1 would be the initial establishment of equilibrium at the core, providing orbital patterns. The 2,3 would be the other portion we know as sun shine, from which wind and other earthy phenomena are explained.

From this thought, I have been able to explain the universe.

It made me exiceted when she said we don't see planets at the exact moment

cue big bang theory theme song

79. vDaBest says:

Why does the intro sound like Watchmojo?

80. Kevin Pears says:

As a real physicist I must ask.. The fact that 80% mass is missing.. Doesn't anyone else wonder if there is a simple error.. There appears to be a problem where Newton is OK is some places but general relativity is OK other places.. Just saying..
I work in materials we have far better success 👍😂

81. FLux says:

So time travel?

82. Daniel Hamilton says:

Bird droppings were used for propellants also known as gun powder derrivative, I do not see what that has to do with them sticking an antenna in the air and discovering the "big bang", but still great video! 🙂 Min 5:54

83. Eunoia says:

Can someone say to me the best course or app for learning physics

84. samir jumaa says:

Waste only God knows not a human with a telescope with a ruler

85. Chris Wallace says:

Ok I'm taking you on a date. Hmu

86. Michael De Stagni says:

why does no one have pupils in this video?

87. Max Boswell says:

I like the way you teach ❤️❤️❤️

88. Rights aren't Wrong says:

In the history of cosmology, astrology and astrophysics there has never been 1 single viable hypothesis. It’s utter pseudoscience. No wonder the flat earthers are owning this debate.

89. Mattias Sjödin says:

I mean if scientists don't know anything about dark energy then why talk about it in absolute terms, as if there are things that can be predicted from its properties

90. Sheldonious says:

Lol it's called the big bang theory for a reason, because it's a THEORY lol, I like how everyone who is ignorant about the research into astrophysics claims the "theory" as fact, there is a higher chance of a monkey typing the dictionary word for word in one go randomly on a typewriter than the big bang occurring and ur saying that is science's #1 theory, even if you can explain the big bang which you can't even still, how did that ball of energy get there that exploded. Like I have no bias it's just sad that people run with this idea like it's Intelligent.

91. Monsier Tippietoes says:

I wanted to become an astrophysicist ever since I was 5, still want to. This video is (sort of) mind blowing.

92. Utkarsh Gautam says:

Are you from indian origin

93. Seize IMmUnITy says:

The way Neil Tyson describes it is Dark matter isnt matter, its unsourcable gravity. Is this true?

94. Isoudine Daher says:

V

95. Restoring Reality says:

What science can know and not know? Physics, 🌎, earth, smart scientists, the way the world works, theorists, taboos, being careful about saying things, what we can say and what we can not, Bull\$hi+! Fullstop 🛑 lying?

Fact! The physics of fluids dictate that liquids like water always find level, every time, without fail, everywhere one might test, regardless of the age or intelligence of the man woman or child to do it, without regard for the feelings of a lost baller in space. The only space that exists lies between us. Earth is Not Flat folks. That is an idiotic assertion! Words mean things. Yes, put them in a looney bin! Obviously there are many differing degrees of ground above and below the sea that is flat level! It is the 💦 water surrounding all the land upon which we stand that is level like the sea.

No nonsense back! i am not a member of your filthy society or clubs. i don't want your insight or require your assistance. Toddlers who have 🎓 graduated from the sippy cup have demonstrated a fundamental understanding of the physics of fluids and you tell me science can't know this! Most of those toddlers, who obviously know more than the totality of the mental capability of knowledge and experience of mankind, many of those babies are still wetting the bed so i firmly assert, piss on scientist and anybody willing to suggest that we should be required to suffer them among us. They are as legions of parasites! No. i will not suffer them! Full 🛑.

96. Daichi Hirata says:

Flames are not plasma. There’s a Ted ed for it.

2:29
How about in case of a distant blackhole?

98. Liane Amorim says:

She speak very fast, is difficult to understand

99. Logan Stewart says:

very epic

100. Vincent Kelly says:

Quantom physics is the answer to this