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billions of stars are disappearing in a in the real sense our horizon is getting smaller because we're able to see fewer things yeah and the strict physics measurement sense it's getting bigger but in a way that kind of doesn't matter yeah millions of stars are becoming unreachable to us every day but what does unreachable mean because we also know that the observable universe is expanding and so we can hypothetically see new stars and galaxies every day i want to investigate what it means for our universe to be getting much bigger and yet much smaller at the same time i want to dig into the most bizarre questions and the realizations that arise when you find out the simple fact that the universe we live in is expanding hey i'm diana you're watching
physics girl and for this video i got in touch with one of the world's leading experts on cosmology dr katie mack and we brought our cameras inside the telescope hubble used to discover the universe was expanding nearly a hundred years ago in my four years at mit i never learned some of the things that we're gonna think about today so it is going to get mind-bending the first crazy thing about cosmology is that we know any of this to begin with and that in less than 100 years ago we didn't even know other galaxies existed let's talk about 1924. it's the decade when people in the u.s had to get cocktails through a secret door in the back of a bowling alley the decade that gave birth to quantum mechanics and the
year that einstein's theory of general relativity turned eight years old and yet we didn't know there were other galaxies we take it for granted now that yeah of course there are other galaxies just do a quick google on your tiny computer in your hand and you'll pull up insanely detailed photos of andromeda check out ngc 6753 or ngc 4696 look at the sombrero galaxy look at it's amazing it's like a roomba exploded but merely a hundred years ago astronomers hebrew curtis and harlow shapley were still screaming at each other over whether the milky way was the entire universe screaming is an exaggeration it's hard to scream the terms peculiar apparent distribution and occulting matter and they were really
debating whether these spiral nebula things that they saw through telescopes were inside the milky way or whether the milky way was really big like 300 000 light years or really small we currently know that it's about 100 000 light years and of course they were debating whether there were other quotes island universes until hubble came along and made a discovery that made shapley say here is the letter that has destroyed my universe so dramatic i love it so i'm researching and looking into this video and i'm thinking where did all of this happen like i like to think about what people were doing and where they were what they smelled like and it turns out the telescope where hubble made his
destructive discovery is in los angeles and i had been there before on accident i camped at this weirdly adorable cabin that you have to hike to and then just a couple miles up the trail i found an observatory that happens to be the most famous observatory in history with its 100-inch telescope where edwin hubble discovered the universe was expanding and so i went back but this of course is the telescope edwin hubble used to discover the universe sounds a little pretentious because you can see the universe people could see it for centuries they knew the universe was there but they didn't know what it was i'm going to take you to that telescope in a second but first a quick message from today's physics girl
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professional visit betterhelp.com physicsgirl that's betterhelp.com physicsgirl to get 10 off your first month and now let's go to the most important telescope in astronomy history and it turns out the telescope where hubble made his discovery is in los angeles and so i went back the mount wilson observatory was founded in 1904 by astronomer george ellery hale that is the hundred inch its main claim to fame the 100-inch telescope was finished in 1917 and was the largest telescope in the world until 1949. to track the sky the entire dome moves which i didn't believe when we were standing inside wait we're moving you're waving oh my gosh that's crazy i completely thought the telescope was moving and we were staying still on the
outside of the dome and i had to walk outside to believe it in the 1900s to the 1920s as some of the most important astronomical discoveries of all time were being made on these telescopes astronomers were probably riding horse-drawn carriages up to the telescopes in modern day cars it takes about an hour to get from hollywood to the mount wilson observatory speaking of cars despite the fact that los angeles is known for its smoggy air there's a really unusual atmospheric phenomenon that creates an inversion layer that keeps the smog trapped at low elevations making this site in the san gabriel mountains one of the clearest for observing in north america in 1917 it was the largest telescope in the world
it is now the largest telescope in all of los angeles county this telescope the moving weight you moved at the moving weights 100 tons the weight is borne by mechanical bearings but to relieve it there's a mercury float so there's mercury down at the bottom at the top so because of these conditions oh and because of the first ever 100 inch telescope whose lens was made out of 9 000 pounds of wine glass hubble was able to discover this very peculiar special thing that you would see if the universe is expanding red shifted galaxies in every direction in the entire universe if the entire universe is expanding which is a very confusing thing to talk about but then that's what you would
expect to see that's the kind of thing albert einstein originally would have predicted from his theory but all the astronomers told him the universe is not expanding and he made the mistake of believing them why did astronomers tell einstein that the universe is not expanding let's accept for a second that it is and think about what that means right now look out into the sky at a speck of light and it may very well be an entire galaxy flying away from us at hundreds of kilometers a second in an expanding universe effectively nearly every galaxy is moving away from us i say nearly because people often ask do we expand with the universe as in our bodies do our atoms spread apart as
spaces created in the universe and the answer is no not even galaxy clusters spread apart and expand apart because the forces keeping our atoms together and the gravity keeping solar systems and galaxies and galaxy clusters together overcome the expansion of space humans are like rhinestones on stretchy pants the rhinestones themselves don't stretch and distant galaxies are like stickers on a slinky put some stickers on my slinky if you stretch the slinky the stickers move apart except that the slinky is just space it's just nothing and more of it gets created which brings up a whole other can of worms about how energy is not conserved on cosmic scales and i have an old video about that
gets interesting about three minutes in if you can make it that far this all means that we are further apart from almost every other galaxy in the universe than we've ever been no wonder scientists had trouble believing this there are places that we've never set foot in the universe and we likely never will because they're moving away from us faster than our fastest rockets that's hard to imagine in a night sky that looks so tranquil and also when historically we had so much trouble accepting that the sun doesn't revolve around the earth that we sent the roman inquisition after galileo now we know that we're just stickers on a slinky but if we're just stickers stuck on a slinky
then what do we even mean by moving do physicists think of this type of moving differently like we say we see distant galaxies moving away from us fast but they're yeah they're not moving and yeah even a spaceship would be moving away from us fast one of them is limited by the speed of light and the other one isn't there at some point you know you can't use the sort of same equations for it um i don't know if you caught that but professor mack just said that one of the types of motion in the universe is not limited by the speed of light from our perspective they appear to be moving away from us faster than the speed of light in actuality those distant galaxies that are beyond that distance where the where the expansion rate makes them faster
than the light they're not really moving and we're not really moving right what's happening is that we're all kind of sitting still and the space in between us is getting bigger the speed of light is the limit for moving through space uh you cannot move through space faster speed of light there's nothing to say that the space in between you and somewhere else can't get bigger in a real quick way right it's a relative thing right like the distance in between us is getting larger such that if something were moving through space in that way it would be moving faster than light but nothing's moving through space itself is getting larger so how did we get from a moment where we didn't even know the universe existed to the
point where we now understand that objects are moving away from us faster than the speed of light it's the early 1920s and edwin hubble is likely sitting in the bitter cold donning one of those fancy man suits he places a photographic glass plate in the focal point of the telescope and is tweaking the joystick rotating the dome so the crosshairs are following his star of choice he's probably thinking of ways that he would overstate his boxing and athletic accomplishments later in life readjusting his cape and he smells like pipe smoke hubble could be a bit of an eccentric bag so at this time hubble knew astronomers were debating the size of the milky way and he too was studying the andromeda nebula as it was called agreeing that
yes it is odd bright specks of light were blinking on and off appearing in and around the nebula were they supernovas why should there be so many more supernovas and stars in front of this nebula as compared to the rest of the sky so that was the mystery of andromeda while he was looking at a specific nova there hubble suddenly realized var seriously this is the most famous correction that he wrote on the most famous astronomical photographic plate in history this one this is a replica but i did get this from my visit to the vault at the carnegie observatory where the original is kept that's amazing over 100 years ago obviously all the parts that are dark would have been bright because this is like a film negative and you can see the
fuzzy part in the middle is the andromeda nebula hubble wrote var and crossed out n because he realized that the nova was actually a cepheid variable which ha exciting variable stars are an amazing phenomenon they blink they make galaxies look alive they actually change in brightness getting dimmer and then brighter as they swell and shrink and hubble could use this to measure distances using levitt's law levitt is henrietta swann levitt an incredibly underappreciated astronomer who basically figured out the first ever equation to reliably measure astronomical distances that was huge i'm guessing you've never heard of her but levitt was so prolific that a member of
the swedish academy of sciences tried to nominate her for a nobel prize levitt was one of those harvard computers one of the women who in the early 1900's was hired to study photographic plates like this one painstakingly laying them over the top of one another looking for minute changes but her boss edward pickering who signed one of her papers as his own probably never imagined that she would go on to discover 2400 variable stars let alone discover the famous period luminosity relationship which turned cepheid variables into a measuring stick if you measure the period with which the star's brightness varies that period tells you what the absolute brightness of the star is at its maximum or minimum if you know
how bright something is and you know how bright it looks then you know how far away from you it is a standard candle so hubble determined the period of his var star that we now call v1 he calculated the luminosity to find the distance before he published it he sent his findings to shapley in a letter and after reading it shapley turned to a colleague and blurted that famous statement here is the letter that has destroyed my universe because the distance that hubble found was 900 000 light years and that put v1 unequivocally outside of the milky way galaxy goosebumps we now know it's two and a half million light years away because we know a lot of stuff hubble didn't know no big surprise there and that was only
his second most significant contribution to history so the universe was bigger than anyone ever imagined every time somebody tries to figure out i'll bet it's that far away they never get it right because it's 10 times farther away the universe is always much bigger than anybody thinks the discovery of andromeda as not a nebula but a galaxy with trillions of stars of its own opened up the boundaries of the cosmos to infinity and beyond until we eventually found essentially the edge of the universe or close to it in 1965 with the detection of the cosmic microwave background and i have another video explaining exactly what that is marginally better than the one on the energy conservation now by the edge of the universe here i
mean the edge of the observable universe and on my way up to mount wilson i had to make sure that my production team knew the difference between the whole universe and the observable universe and so i made them pull over to the side of the road so i was describing how our observable universe is defined by a few things so where you are so i'm going to draw that as like a little point that's where we are and then our observable universe the size of it depends on how fast light travels and how old the universe is so if the universe were older then our observable universe would be out here the observable universe is the part of the universe that we can see uh the part of the universe we can get information from and that boundary is
something like 46 billion light years away in every direction everything we know about the universe suggests that you know the universe carries on far past that there's probably some galaxy you know just right over the border that is beyond our observable universe so we'll never get to see it and the reason that we can't see that galaxy that's beyond our cervical universe is that the light would take so long to get to us from there that even if it started at the very first moment of time it wouldn't have reached us by now so the question is will we ever be able to see that galaxy that's beyond the edge of our observable universe if there is one and the answer is it depends on what you mean by your
question i guess it was my question i realized thinking about the fact that when you look out into space you're looking back in time that the universe is like if you made a circle in photoshop and applied that ugly gradient filter to it where black is now in time and white is in the past all the way to the beginning of the universe but i also realized that there's this other gradient where black is things not moving and white is things moving away from us really fast the universe is really not as simple as it seems the way we observe the universe it's this sphere of time gradient and speed gradient a speed gradient is because of the universe expanding and that's what hubble saw in 1929 hubble took
measurements of the distances to galaxies and then he used data from astronomer vesto schleifer's observations of the redshift of different galaxies throughout the universe but he didn't tell anybody they were schleifer's redshift just published them without acknowledging schleifer hubble you're not the first person to say that and redshift is an indicator of sort of that black to white gradient of how fast something is moving away from us and hubble found a linear relationship he found that more distant galaxies were moving away faster and it was a perfectly linear relationship between how far away the galaxy is to how fast it's moving because as the slinky stretches the galaxies closer to us move slower and the ones further away move
faster look at how fast my hands are moving apart while these two smileys in the middle are barely moving at all so what does this all mean well it means that the observable universe is bigger than it should be whenever i explained it about how it would take the light 13.8 billion years to reach us from the edge of the observable universe people are like oh but you said the observable universe is 46 billion light years in radius why isn't it 13.8 billion light years right yeah if the universe were static it would be if the universe were not expanding at all but because the universe is expanding the point that you know it took 13.8 billion years for the light to reach us that point now is much farther away so
something far enough away can be moving away from us faster than the speed of light that's crazy now what was our original question it was what does it mean for the universe to be getting much bigger and much smaller at the same time there's us and then there's another person over here so their observable universe is going to be um oh god why is this so hard good enough but there's this overlap area right here we can see each other we are in each other's observable universe now imagine and then there's someone over here that person will never see us and we'll never see them and this is a fun little point even though our observable universe is growing this circle is growing time is going on and light can travel
to us from further away so you'd think eventually the circle you know would get it get to be this big and then this big and eventually this person would be within our observable universe the problem is that this is expanding away from us so we may be able to see that galaxy that's outside of the observable universe as it was billions of years ago but we'll never be able to see it as it is now so the observable universe is getting bigger but the number of galaxies within our observable universe is getting smaller yeah technically i mean if you just measure by how far light can travel over the course of time light will keep traveling so that distance will keep getting larger but the expansion of space is getting so
fast that things at that distance are moving away from us too quickly yeah for the light to catch up it's it's a funny thing because there's a lot of stuff out there in the universe that we can see that is currently moving away from us faster than light and in fact always has been we do see some things where the light left them when it was possible for the light to reach us and it is no longer now so we see them as they were in the very distant past we can see them now they will fade away over millions and billions of years and we will never see their future and would not see them as they are now i mean now is a tricky concept in cosmology so this all means we're seeing more and more light from the past but more and more light from the present is
being lost space is expanding that light out of reach forever eventually we learned that not only is the universe expanding but it's accelerating will the universe continue expanding forever you know they knew that gravity would slow the expansion to down so sort of like if you um if you throw a ball up into the air like you give it an initial kick and then it's it's moving up um because you pushed it but it's always slowing down because the gravity of the earth is pulling on it and at some point it'll stop and it'll fall back down to earth what happened in the late 90s is we discovered that the expansion of the universe isn't slowing down at all it's actually speeding up which is analogous to you
know you throw the ball up in the air and it's slowing down for a little while and then it just shoots off into space is that we discovered there's something about space that took over and started expanding the universe faster and faster you know we call that dark energy it might be this cosmological constant which is just a property of space that has this expansion it's kind of built into it if we understood what that was if we really understood the dark energy um we would understand what it's going to do in the future we would know if we are just going to keep accelerating and accelerating in our expansion and you know that leads to one ending of the universe which is called the heat death where
the universe just kind of keeps expanding and keeps expanding and everything kind of cools down and fades away dr mack wrote an entire book on how the universe might end in addition to you know actually researching cosmology so she knows a thing or two about all this the book is amazing you should check it out you know maybe the dark energy is something that changes over time maybe it'll turn around maybe at some point the acceleration will turn around and things will start coming back together then you get a big crunch which is where everything collapses again then there's other possibilities where you have weird kinds of dark energy that can start sort of expanding so fast
it sort of pulls things apart that's a whole other story um so there are a whole bunch a lot of things that have to do with like what is the expansion caused by and what's it gonna do in the future and if we understood what it was caused by if we understood why it's doing what it's doing now that would give us a hint as to what it's going to do in the future i love that the caveat of this is if we understood dark energy which is still something that's completely foreign to us yeah i mean and it's most of the universe you know it's like seventy percent of what the universe is made of right if you carry up like the energy of it and we don't know what it is yeah we don't know what it's going to do yeah that's not disconcerting at all
i would love to see if hubble and einstein and levitt had been alive to find out the universe is not only expanding but it's accelerating that discovery wasn't made until nearly the end of the century in 1998. i think they would be blown away that we're losing stars that we're losing information and i hope that sometime down the road our ancestors are blown away by what we don't know yet thanks for watching and happy physicsing by the way if you're ever in the la area the mount wilson observatory is actually open to the public i highly recommend going
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