Universet er virkelig stort. Vi lever i en galakse, Mælkevejen. Der er omkring et hundrede milliarder stjerner i Mælkevejen. Og hvis man tager et kamera, og man peger det på en tilfældig del af himlen, og man bare holder blænderen åben, så længe ens kamera er fæstnet til Hubble-rumteleskopet vil det se noget som dette. Hver eneste af disse små klatter er en galakse nogenlunde på størrelse med vores Mælkevej -- et hundrede milliarder stjerner i hver af de klatter. Der er omkring et hundrede milliarder galakser i det observerbare univers. 100 milliarder er det eneste tal, man behøver at kende. Universets alder, mellem nu og Big Bang, er et hundrede milliarder i hundeår. (Latter) Hvilket fortæller jer noget om vores plads i universet.
The universe is really big. We live in a galaxy, the Milky Way Galaxy. There are about a hundred billion stars in the Milky Way Galaxy. And if you take a camera and you point it at a random part of the sky, and you just keep the shutter open, as long as your camera is attached to the Hubble Space Telescope, it will see something like this. Every one of these little blobs is a galaxy roughly the size of our Milky Way -- a hundred billion stars in each of those blobs. There are approximately a hundred billion galaxies in the observable universe. 100 billion is the only number you need to know. The age of the universe, between now and the Big Bang, is a hundred billion in dog years. (Laughter) Which tells you something about our place in the universe.
En ting, man kan gøre med et billede som dette, er simpelthen at beundre det. Det er ekstremt smukt. Jeg har ofte undret mig over hvilke evolutionspres, der fik vores forfædre i Velden tilpasse og udvikle sig til virkelig at nyde billeder af galakser, når de ikke havde nogen. Men vi kunne også tænke os at forstå det. Som kosmolog vil jeg gerne spørge, hvorfor er universet sådan her? Et stort spor, vi har, er, at universet forandrer sig med tiden. Hvis man så på en af disse galakser og målte dens hastighed, ville den bevæge sig væk fra en. Og hvis man ser på en galakse endnu længere væk, ville den bevæge sig endnu hurtigere væk. Så vi siger, universet udvider sig.
One thing you can do with a picture like this is simply admire it. It's extremely beautiful. I've often wondered, what is the evolutionary pressure that made our ancestors in the Veldt adapt and evolve to really enjoy pictures of galaxies when they didn't have any. But we would also like to understand it. As a cosmologist, I want to ask, why is the universe like this? One big clue we have is that the universe is changing with time. If you looked at one of these galaxies and measured its velocity, it would be moving away from you. And if you look at a galaxy even farther away, it would be moving away faster. So we say the universe is expanding.
Hvad det betyder er selvfølgelig, at før i tiden var ting tættere på hinanden. Før i tiden var universet tættere, og det var også varmere. Hvis man presser ting sammen, går temperaturen op. Det giver på en måde mening for os. Det, der ikke giver så meget mening for os, er, at universet i tidlige tider nær Big Bang også var meget, meget glat. Man kunne tro, at det ikke er en overraskelse. Luften i dette rum er meget glat. Man kunne sige, "Jamen, måske glattede ting bare sig selv ud." Men vilkårene nær Big Bang er meget, meget forskellige fra vilkårene for luften i dette rum. Især var ting meget tættere. Tingenes gravitationelle træk var meget stærkere nær Big Bang.
What that means, of course, is that, in the past, things were closer together. In the past, the universe was more dense, and it was also hotter. If you squeeze things together, the temperature goes up. That kind of makes sense to us. The thing that doesn't make sense to us as much is that the universe, at early times, near the Big Bang, was also very, very smooth. You might think that that's not a surprise. The air in this room is very smooth. You might say, "Well, maybe things just smoothed themselves out." But the conditions near the Big Bang are very, very different than the conditions of the air in this room. In particular, things were a lot denser. The gravitational pull of things was a lot stronger near the Big Bang.
Det, man er nødt til at tænke på, er, vi har et univers med et hundrede milliarder galakser, et hundrede milliarder stjerner hver. I tidlige tider var de hundrede milliarder galakser presset ind på et område nogenlunde så stort her -- bogstaveligt talt -- i tidlige tider. Og I skal forestille jer, at presse på den måde uden nogen ujævnheder, uden nogen små steder, hvor der var få flere atomer end nogen andre steder. For hvis der havde været det, ville de have kollapset under det gravitationelle træk ind i et stort sort hul. At holde universet meget, meget glat i tidlige tider er ikke let; det er et skrøbeligt arrangement. Det er et spor om, at det tidlige univers ikke blev valgt tilfældigt. Der er noget, der lavede det på den måde. Vi kunne godt tænke os at vide hvad.
What you have to think about is we have a universe with a hundred billion galaxies, a hundred billion stars each. At early times, those hundred billion galaxies were squeezed into a region about this big -- literally -- at early times. And you have to imagine doing that squeezing without any imperfections, without any little spots where there were a few more atoms than somewhere else. Because if there had been, they would have collapsed under the gravitational pull into a huge black hole. Keeping the universe very, very smooth at early times is not easy; it's a delicate arrangement. It's a clue that the early universe is not chosen randomly. There is something that made it that way. We would like to know what.
Så en del af vores forståelse af dette blev givet til os af Ludwig Boltzmann, en østrigsk fysiker i det 19. århundrede. Og Boltzmanns bidrag var, at han hjalp os med at forstå entropi. I har hørt om entropi. Det er tilfældigheden, urodenen, kaosheden i nogle systemer. Boltzmann gav os en formel -- indgraveret på hans gravsten nu -- som virkelig kvantificerer, hvad entropi er. Og den siger basalt set bare, at entropi er antallet af måder, man kan omarrangere et systems bestanddele, så at man ikke ikke kan se det, så at makroskopisk ser ligner det sig selv. Hvis man tager luften i dette rum, lægger man ikke mærke til hvert enkelt atom. En konfiguration med lav entropi er en, i hvilken der kun er få arrangementer, der ser ens ud. Et arrangement med høj entropi er et, som der er mange arrangementer, der ser ens ud. Dette er et afgørende vigtigt indblik, for det hjælper os med at forklare termodynamikkens anden lov -- den lov, der siger, at entropi stiger i universet eller i en isoleret lille del af universet.
So part of our understanding of this was given to us by Ludwig Boltzmann, an Austrian physicist in the 19th century. And Boltzmann's contribution was that he helped us understand entropy. You've heard of entropy. It's the randomness, the disorder, the chaoticness of some systems. Boltzmann gave us a formula -- engraved on his tombstone now -- that really quantifies what entropy is. And it's basically just saying that entropy is the number of ways we can rearrange the constituents of a system so that you don't notice, so that macroscopically it looks the same. If you have the air in this room, you don't notice each individual atom. A low entropy configuration is one in which there's only a few arrangements that look that way. A high entropy arrangement is one that there are many arrangements that look that way. This is a crucially important insight because it helps us explain the second law of thermodynamics -- the law that says that entropy increases in the universe, or in some isolated bit of the universe.
Grunden til, at entropi stiger, er simpelthen fordi, der er mange flere måder at have høj entropi end at have lav entropi på. Det er et vidunderligt indblik, men det udelader noget. Dette indblik, at entropi stiger er forresten det, der er bag det, vi kalder tidens pil, forskellen mellem fortiden og fremtiden. Hver forskel, som der er mellem fortiden og fremtiden, er, fordi entropi stiger -- det faktum, at man kan huske fortiden, men ikke fremtiden. Det faktum, at man bliver født, og så lever man, og så dør man, altid i den rækkefølge, det er fordi, entropi stiger. Boltzmann forklarede, at hvis man starter med lav entropi, er det meget naturligt for den at stige, fordi der er flere måder at få høj entropi på. Det, han ikke forklarede, var, hvorfor entropien nogensinde var lav i første omgang.
The reason why entropy increases is simply because there are many more ways to be high entropy than to be low entropy. That's a wonderful insight, but it leaves something out. This insight that entropy increases, by the way, is what's behind what we call the arrow of time, the difference between the past and the future. Every difference that there is between the past and the future is because entropy is increasing -- the fact that you can remember the past, but not the future. The fact that you are born, and then you live, and then you die, always in that order, that's because entropy is increasing. Boltzmann explained that if you start with low entropy, it's very natural for it to increase because there's more ways to be high entropy. What he didn't explain was why the entropy was ever low in the first place.
Det faktum, at universets entropi var lav, var en refleksion over det faktum, at det tidlige univers var meget, meget glat. Vi kunne godt tænke os at forstå det. Det er vores job som kosmologer. Desværre er det faktisk ikke et problem, som vi har givet nok opmærksomhed. Det er ikke en af de første ting, folk ville sige, hvis man spurgte en moderne kosmolog, "Hvilke problemer forsøger vi at besvare?" En af de folk, der forstod, at dette var et problem, var Richard Feynman. For 50 år siden gav han en serie af en bunke forskellige foredrag. Han gav de populære foredrag, der blev til "The Character of Physical Law." Han gav foredrag til Caltech førsteårsstuderende, som blev til "The Feynman Lectures on Physics." Han gav foredrag til Caltech kandidatstuderende, der blev til "The Feynman Lectures on Gravitation." I hver eneste af disse bøger, hver eneste af disse foredragssæt understregede han denne gåde: Hvorfor havde det tidlige univers så lav en entropi?
The fact that the entropy of the universe was low was a reflection of the fact that the early universe was very, very smooth. We'd like to understand that. That's our job as cosmologists. Unfortunately, it's actually not a problem that we've been giving enough attention to. It's not one of the first things people would say, if you asked a modern cosmologist, "What are the problems we're trying to address?" One of the people who did understand that this was a problem was Richard Feynman. 50 years ago, he gave a series of a bunch of different lectures. He gave the popular lectures that became "The Character of Physical Law." He gave lectures to Caltech undergrads that became "The Feynman Lectures on Physics." He gave lectures to Caltech graduate students that became "The Feynman Lectures on Gravitation." In every one of these books, every one of these sets of lectures, he emphasized this puzzle: Why did the early universe have such a small entropy?
Så han siger -- jeg vil ikke gengive accenten -- han siger, "Af en eller anden grund havde universet på et tidspunkt en meget lav entropi i forhold til dets energiindhold, og siden da er entropien steget. Tidspilen kan ikke blive fuldstændigt forstået før mysteriet om universets histories begyndelse bliver reduceret endnu mere fra spekulation til forståelse." Så det er vores job. Vi vil gerne vide -- dette er for 50 år siden, "Jamen," tænker I, "vi har vel fundet ud af det nu." Det er ikke sandt, at vi har fundet ud af det nu.
So he says -- I'm not going to do the accent -- he says, "For some reason, the universe, at one time, had a very low entropy for its energy content, and since then the entropy has increased. The arrow of time cannot be completely understood until the mystery of the beginnings of the history of the universe are reduced still further from speculation to understanding." So that's our job. We want to know -- this is 50 years ago, "Surely," you're thinking, "we've figured it out by now." It's not true that we've figured it out by now.
Grunden til, at problemet er blevet værre i stedet for bedre er, at i 1998 lærte vi noget meget vigtigt om universet, som vi ikke vidste før. Vi lærte, at det accelererer. Universet udvider sig ikke kun. Hvis man ser på galaksen, bevæger den sig væk. Hvis man kommer tilbage en milliard år senere og ser på den igen, vil den bevæge sig hurtigere væk. Individuelle galakser styrter væk fra os hurtigere og hurtigere, så vi siger, universet accelererer. I modsætningen til det tidlige univers' lave entropi, selvom vi ikke kender svaret på dette, har vi i det mindste en god teori, der kan forklare det, hvis den teori er korrekt, og det er teorien om mørk energi. Det er bare idéen, at tomt rum i sig selv har energi.
The reason the problem has gotten worse, rather than better, is because in 1998 we learned something crucial about the universe that we didn't know before. We learned that it's accelerating. The universe is not only expanding. If you look at the galaxy, it's moving away. If you come back a billion years later and look at it again, it will be moving away faster. Individual galaxies are speeding away from us faster and faster so we say the universe is accelerating. Unlike the low entropy of the early universe, even though we don't know the answer for this, we at least have a good theory that can explain it, if that theory is right, and that's the theory of dark energy. It's just the idea that empty space itself has energy.
I hver lille kubikcentimeter rum, uanset om der er noget eller ej, uanset om der er partikler, stof, stråling eller andet eller ej, er der stadig energi, selv i rummet selv. Og denne energi udøver ifølge Einstein et skub på universet. Det er en uophørlig impuls, der skubber galakser væk fra hinanden. For mørk energi, i modsætningen til stof eller stråling, tynder ikke ud som universet udvider sig. Mængden af energi i hver kubikcentimeter forbliver den samme, selv som universet bliver større og større. Dette har meget vigtige implikationer for, hvad universet vil gøre i fremtiden. For det første vil universet udvide sig for evigt.
In every little cubic centimeter of space, whether or not there's stuff, whether or not there's particles, matter, radiation or whatever, there's still energy, even in the space itself. And this energy, according to Einstein, exerts a push on the universe. It is a perpetual impulse that pushes galaxies apart from each other. Because dark energy, unlike matter or radiation, does not dilute away as the universe expands. The amount of energy in each cubic centimeter remains the same, even as the universe gets bigger and bigger. This has crucial implications for what the universe is going to do in the future. For one thing, the universe will expand forever.
Dengang da jeg var på jeres alder, vidste vi ikke, hvad universet ville gøre. Nogle folk troede, at universet ville falde sammen igen i fremtiden. Einstein var tilhænger af denne idé. Men hvis der er mørk energi, og den mørke energi ikke forsvinder, vil universet bare fortsætte med at udvide sig for evigt og altid og altid. 14 milliarder år i fortiden, 100 milliarder hundeår, men et uendeligt antal år i fremtiden. Imens, til alle hensigter og formål, ser rummet endeligt ud for os. Rummet kan være endeligt eller uendeligt, men fordi universet accelererer, er der dele af det vi ikke kan se og aldrig vil se. Der er en endelig del af rummet, som vi har adgang til, omringet af en horisont. Så selv om tiden fortsætter for evigt, er rummet begrænset for os. Endelig har rum en temperatur.
Back when I was your age, we didn't know what the universe was going to do. Some people thought that the universe would recollapse in the future. Einstein was fond of this idea. But if there's dark energy, and the dark energy does not go away, the universe is just going to keep expanding forever and ever and ever. 14 billion years in the past, 100 billion dog years, but an infinite number of years into the future. Meanwhile, for all intents and purposes, space looks finite to us. Space may be finite or infinite, but because the universe is accelerating, there are parts of it we cannot see and never will see. There's a finite region of space that we have access to, surrounded by a horizon. So even though time goes on forever, space is limited to us. Finally, empty space has a temperature.
I 1970'erne fortalte Stephen Hawking os, at et sort hul, selvom man tror det er sort, faktisk afgiver stråling, når man tager kvantemekanik med i overvejelserne. Krumningen af rumtiden omkring det sorte hul bringer de kvantemekaniske fluktuationer til live, og det sorte hul stråler. En præcist lignende udregning af Hawking og Gary Gibbons viste, at hvis man har mørk energi i tomt rum, så stråler hele universet. Det tomme rums energi bringer kvantefluktuationer til live. Og derfor, selvom universet vil vare for evigt, og normalt stof og stråling vil blive udvandet til det forsvinder, vil der altid være noget stråling, nogle termiske fluktuationer, selv i tomt rum. Så det, det her betyder, er, at universet er som en kasse med gas, der varer evigt. Nå, hvad er implikationen af det?
In the 1970s, Stephen Hawking told us that a black hole, even though you think it's black, it actually emits radiation when you take into account quantum mechanics. The curvature of space-time around the black hole brings to life the quantum mechanical fluctuation, and the black hole radiates. A precisely similar calculation by Hawking and Gary Gibbons showed that if you have dark energy in empty space, then the whole universe radiates. The energy of empty space brings to life quantum fluctuations. And so even though the universe will last forever, and ordinary matter and radiation will dilute away, there will always be some radiation, some thermal fluctuations, even in empty space. So what this means is that the universe is like a box of gas that lasts forever. Well what is the implication of that?
Den implikation blev studeret af Boltzmann tilbage i det 19. århundrede. Han sagde, altså, entropi stiger, fordi der er mange, mange flere måder for universet at have høj entropi end at have lav entropi. Men det er en probabilistisk udtalelse. Den vil sandsynligvis stige, og sandsynligheden er enormt stor. Det er ikke noget, man skal bekymre sig om -- at luften i dette rum alt sammen skulle samle sig ovre i én del af rummet og kvæle os. Det er meget, meget usandsynligt. Bortset fra hvis de låste dørene og holdt os her bogstaveligt talt for evigt, ville det ske. Alt, der er tilladt, enhver konfiguration, det er tilladt for molekylerne at opnå i dette rum, ville eventuelt blive opnået.
That implication was studied by Boltzmann back in the 19th century. He said, well, entropy increases because there are many, many more ways for the universe to be high entropy, rather than low entropy. But that's a probabilistic statement. It will probably increase, and the probability is enormously huge. It's not something you have to worry about -- the air in this room all gathering over one part of the room and suffocating us. It's very, very unlikely. Except if they locked the doors and kept us here literally forever, that would happen. Everything that is allowed, every configuration that is allowed to be obtained by the molecules in this room, would eventually be obtained.
Så Boltzmann siger, altså, man kunne starte med et univers, der var i termisk ligevægt. Han kendte ikke til Big Bang. Han kendte ikke til universets udvidelse. Han troede, at rum og tid blev forklaret af Isaac Newton -- de var absolutte; de sad der bare for evigt. Så hans opfattelse af et naturligt univers var en, hvori luftmolekylerne bare var spredt jævnt ud overalt -- alt-molekylerne. Men hvis man er Boltzmann, ved man, at hvis man venter længe nok, vil de molekylers tilfældige fluktuationer af og til bringe dem ind i konfigurationer med lavere entropi. Og så, selvfølgelig, efter tingenes naturlige rækkefølge, vil de udvide sig tilbage. Så det er ikke fordi, entropi altid skal stige -- man kan få fluktuationer til lavere entropi, mere organiserede situationer.
So Boltzmann says, look, you could start with a universe that was in thermal equilibrium. He didn't know about the Big Bang. He didn't know about the expansion of the universe. He thought that space and time were explained by Isaac Newton -- they were absolute; they just stuck there forever. So his idea of a natural universe was one in which the air molecules were just spread out evenly everywhere -- the everything molecules. But if you're Boltzmann, you know that if you wait long enough, the random fluctuations of those molecules will occasionally bring them into lower entropy configurations. And then, of course, in the natural course of things, they will expand back. So it's not that entropy must always increase -- you can get fluctuations into lower entropy, more organized situations.
Jamen hvis det er sandt, går Boltzmann så videre til at opfinde to idéer, der lyder meget moderne -- multiverset og det antropiske princip. Han siger, problemet med termisk ligevægt er, at vi ikke kan leve der. Husk, livet selv afhænger af tidens pilen. Vi ville ikke være i stand til at bearbejde information, fordøje, gå og tale, hvis vi levede i termisk ligevægt. Så hvis man forestiller sig et meget, meget stort univers, et uendeligt stort univers med partikler, der tilfældigt støder ind i hinanden, vil der af og til være små fluktuationer til tilstande med lavere entropi, og så falder de tilbage igen. Men der vil også være store fluktuationer. Af og til vil man lave en planet eller en stjerne eller en galakse eller et hundrede milliarder galakser. Så Boltzmann siger, vi kun vil leve i den del af multiverset, i den del af dette uendeligt store sæt af fluktuerende partikler, hvor liv er muligt. Det er den region, hvor entropi er lav. Måske er vores univers bare en af disse ting, der sker fra tid til anden.
Well if that's true, Boltzmann then goes onto invent two very modern-sounding ideas -- the multiverse and the anthropic principle. He says, the problem with thermal equilibrium is that we can't live there. Remember, life itself depends on the arrow of time. We would not be able to process information, metabolize, walk and talk, if we lived in thermal equilibrium. So if you imagine a very, very big universe, an infinitely big universe, with randomly bumping into each other particles, there will occasionally be small fluctuations in the lower entropy states, and then they relax back. But there will also be large fluctuations. Occasionally, you will make a planet or a star or a galaxy or a hundred billion galaxies. So Boltzmann says, we will only live in the part of the multiverse, in the part of this infinitely big set of fluctuating particles, where life is possible. That's the region where entropy is low. Maybe our universe is just one of those things that happens from time to time.
Nå jeres hjemmeopgave er virkelig at tænke over dette, at overveje hvad det betyder. Carl Sagan sagde som bekendt engang, at "for at lave en æbletærte, må man først opfinde universet." Men han tog fejl. I Boltzmanns scenarie hvis man vil lave en æbletærte, venter man bare på, at atomernes tilfældige bevægelser laver en æbletærte til en. Det vil ske meget oftere, end at atomernes tilfældige bevægelser laver en æbleplantage til en og så noget sukker og en ovn, og så laver en æbletærte til en. Så dette scenario laver forudsigelser. Og forudsigelserne er, at fluktuationerne, der laver os, er minimale. Selv hvis man forestiller sig, dette rum, vi er i nu, eksisterer og er virkeligt, og her er vi, og vi har ikke bare vores minder, men vores indtryk, at udenfor er der noget, der hedder Caltech og USA og Mælkevejen, er det meget lettere for alle disse indtryk tilfældigt at fluktuere inde i ens hjerne, end at de rent faktisk tilfældigt fluktuerer til Caltech, USA og galaksen.
Now your homework assignment is to really think about this, to contemplate what it means. Carl Sagan once famously said that "in order to make an apple pie, you must first invent the universe." But he was not right. In Boltzmann's scenario, if you want to make an apple pie, you just wait for the random motion of atoms to make you an apple pie. That will happen much more frequently than the random motions of atoms making you an apple orchard and some sugar and an oven, and then making you an apple pie. So this scenario makes predictions. And the predictions are that the fluctuations that make us are minimal. Even if you imagine that this room we are in now exists and is real and here we are, and we have, not only our memories, but our impression that outside there's something called Caltech and the United States and the Milky Way Galaxy, it's much easier for all those impressions to randomly fluctuate into your brain than for them actually to randomly fluctuate into Caltech, the United States and the galaxy.
De gode nyheder er, at derfor virker dette scenario ikke; det er ikke rigtigt. Dette scenario forudser, at vi skulle være en minimal fluktuation. Selv hvis man fjernede vores galakse, ville man ikke få hundrede milliarder andre galakser. Og Feynman forstod også dette. Feynman siger, "Fra hypotesen, at verden er en fluktuation, er alle forudsigelserne, at hvis vi ser på en del af verden, vi aldrig har set før, vil vi se den som blandet, og ikke som delen vi lige så på -- høj entropi. Hvis vores orden skyldtes en fluktuation, ville vi ikke forvente orden nogen andre steder end der, vi lige har bemærket det. Vi konkluderer derfor, at universet ikke er en fluktuation." Så det er godt. Spørgsmålet er så, hvad er det rigtige svar? Hvis universet ikke er en fluktuation, hvorfor havde det tidlige univers lav entropi? Og jeg ville elske at give jer svaret, men jeg er ved at løbe tør for tid.
The good news is that, therefore, this scenario does not work; it is not right. This scenario predicts that we should be a minimal fluctuation. Even if you left our galaxy out, you would not get a hundred billion other galaxies. And Feynman also understood this. Feynman says, "From the hypothesis that the world is a fluctuation, all the predictions are that if we look at a part of the world we've never seen before, we will find it mixed up, and not like the piece we've just looked at -- high entropy. If our order were due to a fluctuation, we would not expect order anywhere but where we have just noticed it. We therefore conclude the universe is not a fluctuation." So that's good. The question is then what is the right answer? If the universe is not a fluctuation, why did the early universe have a low entropy? And I would love to tell you the answer, but I'm running out of time.
(Latter)
(Laughter)
Her er universet, som vi fortæller jer om, versus universet, der virkelig eksisterer. Jeg har lige vist jer dette billede. Universet har udvidet sig i de sidste 10 milliarder år sådan ca. Det køler af. Men vi ved nu nok om universets fremtid til at sige meget mere. Hvis mørk energi fortsætter med at være tilstede, vil stjernerne omkring os opbruge deres kernebrændsel, de vil stoppe med at brænde. De vil falde sammen til sorte huller. Vi vil leve i et univers med intet i sig ud over sorte huller. Det univers vil vare 10 opløftet i 100 år -- meget længere end vores lille univers har levet. Fremtiden er meget længere end fortiden. Men selv sorte huller varer ikke evigt. De vil fordampe, og vi vil blive tilbage med intet ud over tomt rum. Det tomme rum varer grundlæggende set evigt. Men I bemærker, eftersom tomt rum afgiver stråling, er der faktisk termiske fluktuationer, og det kører i ring alle de forskellige mulige kombinationer af grader af frihed, der eksisterer i tomt rum. Så selvom universet varer evigt, er der kun et endeligt antal af ting, der kan lade sig gøre i universet. De sker allesammen over en tidsperiode på 10 opløftet i 120 år.
Here is the universe that we tell you about, versus the universe that really exists. I just showed you this picture. The universe is expanding for the last 10 billion years or so. It's cooling off. But we now know enough about the future of the universe to say a lot more. If the dark energy remains around, the stars around us will use up their nuclear fuel, they will stop burning. They will fall into black holes. We will live in a universe with nothing in it but black holes. That universe will last 10 to the 100 years -- a lot longer than our little universe has lived. The future is much longer than the past. But even black holes don't last forever. They will evaporate, and we will be left with nothing but empty space. That empty space lasts essentially forever. However, you notice, since empty space gives off radiation, there's actually thermal fluctuations, and it cycles around all the different possible combinations of the degrees of freedom that exist in empty space. So even though the universe lasts forever, there's only a finite number of things that can possibly happen in the universe. They all happen over a period of time equal to 10 to the 10 to the 120 years.
Så her er to spørgsmål til jer. Nummer et: Hvis universet varer i 10 opløftet i 10 opløftet i 120 år, hvorfor er vi født i de første 14 milliarder år af den tid i Big Bangs varme, behagelige efterglød? Hvorfor lever vi ikke i tomt rum? I kunne sige, "Jamen, der er ingenting til at leve," men det er ikke rigtigt. I kunne være en tilfældig fluktuation ud af intetheden. Hvorfor er I ikke? Mere hjemmearbejde til jer.
So here's two questions for you. Number one: If the universe lasts for 10 to the 10 to the 120 years, why are we born in the first 14 billion years of it, in the warm, comfortable afterglow of the Big Bang? Why aren't we in empty space? You might say, "Well there's nothing there to be living," but that's not right. You could be a random fluctuation out of the nothingness. Why aren't you? More homework assignment for you.
Så som jeg sagde, kender jeg faktisk ikke svaret. Jeg vil give jer mit yndlingsscenario. Enten er det bare sådan. Der er ingen forklaring. Dette er et hårdt faktum om universet, som man bør lære at acceptere og stoppe med at stille spørgsmål. Eller måske er Big Bang ikke universets begyndelse. Et æg, et ubrudt æg, er en konfiguration med lav entropi, og når vi alligevel åbner vores køleskab, siger vi ikke, "Hah, hvor overraskende at finde denne konfiguration med lav entropi i vores køleskab." Det er fordi et æg ikke er et lukket system; det kommer fra en høne. Måske kommer universet fra en universal høne. Måske er der noget, der helt naturligt, ved vækst fra fysikkens love, giver anledning til et univers som vores med konfigurationer med lav entropi. Hvis det er sandt, ville det ske mere end én gang; vi ville være del af et meget større multivers. Det er mit yndlingsscenario.
So like I said, I don't actually know the answer. I'm going to give you my favorite scenario. Either it's just like that. There is no explanation. This is a brute fact about the universe that you should learn to accept and stop asking questions. Or maybe the Big Bang is not the beginning of the universe. An egg, an unbroken egg, is a low entropy configuration, and yet, when we open our refrigerator, we do not go, "Hah, how surprising to find this low entropy configuration in our refrigerator." That's because an egg is not a closed system; it comes out of a chicken. Maybe the universe comes out of a universal chicken. Maybe there is something that naturally, through the growth of the laws of physics, gives rise to universe like ours in low entropy configurations. If that's true, it would happen more than once; we would be part of a much bigger multiverse. That's my favorite scenario.
Så arrangørerne bad mig om at slutte med en dristig spekulation. Min dristige spekulation er, at jeg bliver fuldstændigt bekræftet engang. Og om 50 år fra nu bliver alle mine lige nu vilde idéer accepteret som sandheder af de videnskabelige og eksterne samfund. Vi vil alle tro på, at vores lille univers bare er en lille del af et meget større multivers. Og endnu bedre vil vi forstå det, der skete ved Big Bang med en teori, som vi vil være i stand til at sammenligne med observationer. Dette er en forudsigelse. Jeg kunne tage fejl. Men vi har tænkt som menneskerace over, hvordan universet var, hvorfor det blev på den måde, det blev i mange, mange år. Det er spændende at tænke på, vi måske endelig kommer til at kende svaret engang.
So the organizers asked me to end with a bold speculation. My bold speculation is that I will be absolutely vindicated by history. And 50 years from now, all of my current wild ideas will be accepted as truths by the scientific and external communities. We will all believe that our little universe is just a small part of a much larger multiverse. And even better, we will understand what happened at the Big Bang in terms of a theory that we will be able to compare to observations. This is a prediction. I might be wrong. But we've been thinking as a human race about what the universe was like, why it came to be in the way it did for many, many years. It's exciting to think we may finally know the answer someday.
Tak.
Thank you.
(Bifald)
(Applause)