Can We Observe the Origin of Structure in the Universe?
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00:00
DestroyerChandrasekhar limitJosephson-KontaktSkyUniverseSpeckle imagingEmissionsvermögenGalaxyApertureProzessleittechnikRadio galaxyCrystal structureDensityElementary particleSynchrotronWolkengattungFunkgerätBig BangSource (album)Radio astronomyHot workingDirect currentGaussian beamQuasarElliptischer NebelVideoBlack holeRutschungSignal (electrical engineering)YearOrbital periodRadio telescopeMeasuring instrumentPower (physics)Impact eventSpace probeNumber densityMaterialPhase (matter)Beam emittanceRRS DiscoveryRoll formingLastDrehmasseRedshiftDayMeeting/Interview
07:57
BoatDestroyerChandrasekhar limitJosephson-KontaktGroup delay and phase delayVakuumphysikBig BangNegative pressureAtomic nucleusElementary particleAngeregter ZustandUniverseFire apparatusYearCosmic microwave background radiationRestkernGalaxyModel buildingForceTemperatureAntenna (radio)StrangenessRadiationPhase (matter)NeutrinoCrystal structureCombined cycleFormation flyingPair productionFundamental frequencyAlcohol proofOrbital periodLightSkyBahnelementProzessleittechnikEngineDensityCartridge (firearms)Direct currentRoll formingSignal (electrical engineering)Enclosure (electrical)Nuclear powerRadio astronomyRadio telescopeRadio galaxyAtomismHydrogen atomPhotonicsCommunications satelliteFACTS (newspaper)MicrowaveCondenser (heat transfer)Condensed matter physicsRRS DiscoveryMeeting/Interview
15:54
BoatDestroyerChandrasekhar limitJosephson-KontaktMinuteGroup delay and phase delayAmpouleCosmic microwave background radiationGround (electricity)Dark matterMeasuring instrumentElementary particleRutschungScale (map)Cluster (physics)UniverseRelative articulationYearRadiationCrystal structureDensityGalaxyExotic atomMassRoll formingAlcohol proofQuantum fluctuationOrbitSkyPhase (matter)ConcentratorHubble's lawHydrogen atomRadio telescopeFormation flyingPair productionPerturbation theoryDayBig CrunchMeasurementNeutronOrder and disorder (physics)TemperatureNumber densityEffects unitDirect currentModel buildingSpare partPlanetRotationSpantWire bondingBrightnessSchwache LokalisationSpeckle imagingStarSunlightReference workKelvinPhotonicsBlast furnace
23:51
Group delay and phase delayFACTS (newspaper)Cluster (physics)Effects unitUniverseGround (electricity)Alcohol proofApparent magnitudeArc lampAbsorption (electromagnetic radiation)Cosmic microwave background radiationString theoryRadiationAstronomerDrehmasseElectronQuantum fluctuationMicrowaveMeasuring instrumentReceiver (radio)Crystal structureVerband Schweizer Abwasser- und GewässerschutzfachleuteRadio telescopeCompton scatteringSkyVisibilityMapVakuumphysikGasPerturbation theoryRoll formingScale (map)Enclosure (electrical)Line-of-sight propagationRadio astronomySteckverbinderArray data structurePhotonicsModel buildingActive laser mediumCondenser (heat transfer)GalaxyDark matterRadio galaxyNegativer WiderstandAudio frequencyPhase (matter)Formation flyingAngeregter ZustandElectronic componentRedshiftDirect currentTroposphäreMonthYearVortexTemperatureSunriseSignal (electrical engineering)Energy levelParticle physicsOrder and disorder (physics)Bandwidth (signal processing)MinuteWolkengattungSchwache LokalisationMeasurementRoots-type supercharger
Transcript: English(auto-generated)
00:13
but I believe that similar methods will be followed in other places. So, for those of you who are young scientists beginning your careers in research,
00:26
I hope that some of the ideas I shall be suggesting may be those places where you can find satisfying research. When you're a young scientist, you should not be following fashionable research.
00:41
The main trends in a subject as old as radio astronomy may be only a little only 30 years old, but nevertheless the main lines of the subject are now quite well defined and there are schools of thought and instruments which are well established and it is so easy to go into a conventional field
01:02
where most of the spade work has been done and what remains is fascinating but nevertheless not so fundamental perhaps as some original work in another area. I believe what I'm talking about is such an area. The theoreticians are, I might say, floundering.
01:23
There are many confusing and conflicting ideas as I shall explain and this is exactly the area for a young observer to enter. It is really very nice to be able to say which theories are right and which theories are wrong and this is the aim of an observational scientist.
01:44
Well now, what are the problems in cosmology? Well, radio astronomy has had a major impact and has changed our thinking in two major ways and I can illustrate this very briefly. May I have the first slide please?
02:02
Radio galaxies. The discovery of radio galaxies in the early days of radio astronomy the moment we realized that these objects had very high redshifts it was clear that the radio galaxies, these amazing images
02:20
which we can now obtain with the best modern radio telescopes we realized that they, long ago, that they were ideal probes for investigating the past history of the universe. They are so far away that signals from them have crossed space for many several thousand million years
02:43
a time comparable to the history of the whole universe if one believes in a Big Bang cosmology and therefore these are ideal ways, ideal probes which tell you something about the behavior of the universe at an earlier phase in its history.
03:01
Here you see an image of a typical radio galaxy which we made at Cambridge using the last major radio telescope designed by Martin Ryle. He invented the technique, which is now universal of aperture synthesis, which enables one to make high resolution images of these extremely distant objects.
03:22
Now, what you see there is a computer map of the radio emission from not such a distant radio galaxy but you see it has a dot in the middle and if you were to look with an optical telescope in that direction, you would see there a galaxy probably an elliptical galaxy
03:41
but the radio galaxy is very much larger and it is quite clear that energy is being transported outwards along those snake-like paths and is being released in clouds at the ends where we have relativistic particles which emit the synchrotron radio emission which causes these galaxies to be detectable.
04:03
Now, there is much of interest in radio galaxies still to be discovered. We do not know what the source of energy is. Probably in the centers of many galaxies we find active nuclei, compressed matter near the center of a galaxy
04:20
which may well be in the form of a black hole and energy is being released from this in well-defined directions. The energy is probably in the form of a relativistic beam of particles most likely electron-positron pairs but whatever is the source of energy we now know roughly the physical processes
04:41
which go on in a radio galaxy It is because they are the most powerful emitters of energy in the sky that we can use them for these cosmological investigations which I mentioned. May I have the second slide? I'll show you one more example of a radio galaxy. The next slide, please.
05:02
This is an even more peculiar object mapped by a radio telescope in the United States and you can see again the central dot which is the source of power and you can see the narrow directed beams of energy which seem to be puffing clouds of particles outwards in each direction.
05:22
This is typical behavior of radio galaxies and the mystery is in that dot in the center when one looks with optical telescopes one frequently sees a spot of light these were therefore frequently called quasars quasi-stellar objects the images I've shown you
05:41
have been of relatively nearby ones where you can see the structure quite clearly. Well, these are the typical objects radio galaxies and what is their importance for cosmology? Well, when radio astronomy began it wasn't certain that the universe was static in the sense that over periods of time
06:01
its structure didn't change there was a popular cosmological theory called the steady state cosmology as the universe expanded so material was created to fill the spaces and in that sense the universe would be like a population of people the people changing from time to time
06:21
but the population remaining about the same that was a very nice theory and was very popular but of course if one can look back over the history of the universe then one is able to test this assumption is the universe constant in time or not and this was work started by Martin Ryle in Cambridge and elaborated by many others
06:42
it's now very clear that the universe as one observes it back in time is very different from the universe today simply if we count the radio galaxies in the sky this is a recent graph made by Malcolm Longer in Edinburgh in the United Kingdom
07:00
and it plots simply the density the number density of radio galaxies as one looks back from the present time here which I call now to the beginning of the universe perhaps around 10 to the 10 10 billion years ago and it is possible from the observations we have to predict that the density
07:21
of these radio galaxies must have been very much higher early in the universe we can look back over history over roughly 9 tenths of the history of the whole universe if we take it to be a big bang universe created at some instant and as you can see the numbers increase here at the very early phases of the universe
07:42
radio galaxies were some hundred to a thousand times more numerous in a given volume of space as compared to today the universe is evolving in a very major way and this became evident in the late 1950s, the early 1960s
08:01
it was clear that continuous creation a steady state universe with no beginning or no end was impossible it had to be evolving and of course this led straight to the so called big bang cosmologies which of course were models which had been much much considered well now radio galaxies are one of the major features
08:22
of radio astronomy but the feature I want to be talking more about today results from the discovery of Penzias and Wilson here is their apparatus Penzias and Wilson as I'm sure you know discovered that the sky was filled with a very weak background
08:42
radiation corresponding to thermal radiation at a temperature of a little under 3 degrees Kelvin here is the apparatus they used it looks a little strange but in fact it's a very well designed radio telescope it was designed not for astronomy but for satellite communication
09:00
engineering but those scientists discovered a weak radiation coming in all directions from the sky which they could not explain in any other way than that it was an extraterrestrial signal the antenna here you see is open to the sky and it was inside the antenna was quite a popular place
09:21
for birds to make their nests it was a little warmer in there than outside and luckily they did not actually affect the experiment very much now the only reasonable explanation it appears is that this radiation this microwave radiation
09:42
is heat radiation left over from an early hot phase of the universe and it is immensely important in cosmology because if one takes the the models which relate to the expanding universe considered under the theory of general
10:00
relativity models of the universe such as the Einstein De Sitter universe expanding steadily if we know that it is filled with this radiation then we can compute what the universe was like at earlier times the evolution of the universe is something now which we can consider physically
10:22
in a quantitative way and in the time available I can only hint at the structure we observe, but if we work back over the universe from the universe we see today which is filled with galaxies and has this weak background radiation of around 3 degrees Kelvin filling it
10:41
then at earlier times it must have been hotter of course if you put radiation in an enclosure, in this case the enclosure is the universe and you compress it adiabatically the temperature will rise and this is essentially what happens in the universe it expands adiabatically from a hot condensed phase
11:01
to a much lower temperature lower density phase and we can work out from the ordinary laws of physics what the matter in it must have been like there are many problems in relating one phase to another, but if we look back from the present time to, for example
11:20
a few seconds from the beginning of everything then we can relate a temperature to each time and at a temperature, say of around 10 seconds temperature is around 10 to the 10 degrees Kelvin and this is the region where we can just begin to form atomic nuclei earlier than that
11:41
around 1 microsecond from the origin of time I think it's still amazing that we can discuss the physical structure of the universe in terms of conventional laboratory physics only one millionth of a second after the universe was created but nevertheless that seems to be true at that time it was so hot
12:01
temperature of 10 to the 12 Kelvin that conglomerations of particles are no longer possible you cannot have simple nuclei you have to have individual particles neutrons, protons, neutrinos and so forth at earlier times things become harder to understand and we need more physics the idea, the hope is that at even earlier times
12:22
like 10 to the minus 30 seconds from the beginning there is a theory we call it grand unified theory which accounts for all the forces in a homogeneous way there is no distinction one hopes between the fundamental forces of nature at a sufficiently early time and this is the aim
12:41
of grand unified theories which ultimately one hopes will combine quantum physics and general relativity that of course is a goal we are not too close to that goal if those theories are indeed correct the kind of theories now being formulated then the universe went through a very exciting phase
13:02
even before 10 to the minus 30 of a second when no normal particle would be possible the universe can be in a state the quantum physicists call it a false vacuum where the vacuum energy dominates the situation and the universe simply expands rapidly under the effect of
13:22
a negative pressure it is a strange idea but general relativity predicts that a negative pressure in the universe causes it to expand and under those conditions it can expand increasing its energy at the same time if you expand a volume with negative pressure of course you gain energy and this is the way the universe may have begun
13:42
it is called the theory of inflation and is a very popular idea at present it solves a number of problems now this picture here seems maybe a little far from reality how can we be sure that the universe is anything like our predictions using physical theory well of course there is much evidence
14:01
the period of the formation of simple atomic nuclei at a few tens of seconds after creation that is something we know very well what the physics of it is and we can predict the quantities of light light elements that will be created and the interesting feature is that we make observations of the universe
14:21
we can calculate the amount of hydrogen, helium lithium, deuterium and other things and it appears that our calculations are well explained by this model of the time the universe spent in passing through at a particular density and temperature the phase of nuclear formation
14:40
and the observations fit the theory very well and it is rather hard to see how this general scheme could be incorrect it's much simpler to believe that this is how the elements originate than to dream up some other method of course the further back we go the harder it gets we can't get back beyond one microsecond
15:02
well now, what is the importance of this for the radio astronomy that I was talking about we see a universe filled with galaxies and it is sad to say that we just don't know how those galaxies arose clearly in a hot expanding universe we have to have gravitational condensation
15:21
of matter and this takes a certain amount of time well there's a problem and this is the experiment I shall be talking about how do we go how do we understand the processes which operate from the beginning to ten to the ten years to produce the wonderful structure of the galaxies in the sky that we see today
15:41
a critical time is ten to the five years after creation which is when the simple atoms of hydrogen form and it is at this time that the universe first becomes transparent clearly if photons are scattered in their passage through the universe we can only see a blurred image it's rather like being inside a furnace
16:01
you do not see any structure but when photons can travel freely across the universe we can then begin with instruments on the ground to make cosmological observations and the radiation we pick up now as the microwave background at a few degrees Kelvin was the radiation which was present after about ten to the five years from
16:21
creation when the universe had a temperature of a few thousand degrees Kelvin now the interesting thing is that this radiation is amazingly constant over the sky the radiation is constant over the sky as we see it in different directions to about
16:41
certainly better than one part in ten to the four if we correct for the motion of our own planet through the universe if you are in a body, if you are surrounded by blackbody radiation and you move through it at a certain speed then relativity theory tells you quite clearly that the radiation will appear
17:01
hotter towards the direction in which you are moving and cooler in the reverse direction and this is very readily measurable with accurate radio telescopes we can see that the Earth our local frame of reference is moving through the universe at a speed of about 350
17:21
km per second. It is also possible to measure the speed of the Earth around the Sun looking at the blackbody radiation such as the accuracy of present day measurements. Well now apart from that this is a big effect which we expect because different parts of the universe are not expanding uniformly with the Hubble expansion. They have their own motion
17:41
due to perhaps the rotation of the galaxy we inhabit and the motion of that galaxy relative to others and large scale motions all these things What then is the do we know about the large scale structure of the universe May I have the next slide please
18:00
I don't know if you can see this but the universe as seen through an optical telescope is structured in the sense that the galaxies form clusters. We have a hierarchical system I'm not sure that it fits the fractal picture too well but the stars that make the galaxies and the galaxies themselves are clustered as you see in this slide here
18:21
and the general structure of the universe on a large scale seems to be clusters of galaxies these are the main features If one wants to see this on a larger scale then one has to look at pictures like this This is the northern sky and
18:41
shows you a map from the famous Lick survey in the 1960s which gives you the density, the number density of galaxies in pixel formation of just around one million galaxies and the structure you see here is clearly non-uniform. It's a little hard to specify exactly how non-uniform it is but one sees places where
19:01
galaxies are clustered and places where galaxies are rather thinly spread and this is on a structure of about speaking astronomically 150 mega parsecs or something like 300 million light years a few hundred million light years this is the basic structure which the universe
19:21
contains and we know that these clusters of galaxies are gravitationally bound. We see them moving in orbits about each other and this is the structure we have to explain Well, how does it come about? If the universe is very uniform at ten to the five years
19:41
then density concentrations have to form and condense into galaxies in the time available which is of the order of a few times ten to the nine years and this is where there's a problem because if we look at the large scale universe as I've just shown you and ask
20:00
what density fluctuations should have been present in the universe at ten to the five years after creation then it is quite clear that to allow the galaxies to form one needs densities of the order of density fluctuations of 0.1% about one part in ten to the three and we know that the density fluctuations
20:22
we can observe because they would show up as fluctuations of brightness in the microwave background we know that those are ten times at least ten times weaker than that so we have no reason to see galaxies in the sky there is not time to have made them how can they have actually appeared?
20:42
Well, we have primordial fluctuations of density which one can certain theories can predict but they seem to be insufficient we seem to see insufficient of them to cause the structure of the universe observed around us one way out of this is to suppose that there is more matter in the universe
21:01
than we can currently observe now we know already that perhaps this is a bad admission for an astrophysicist but we can only detect less than 10% of all the matter we know to exist in the sky we see galaxies and we can infer from them the mass of the system they inhabit and if we make extrapolations
21:22
from these measurements which are not too inaccurate we can compute that the universe contains so much total mass well now, when we do that we find there is mass which is present from dynamical considerations we see galaxies orbiting a central concentration of mass which is invisible
21:40
we know that there is about ten times as much mass present in clusters of galaxies as there is in the galaxies that we can see directly by stars this is a famous problem in astrophysics the problem of dark matter or unseen matter but when it comes to galaxy formation we would like even more matter
22:00
because it is possible for galaxies to form if the universe contains roughly what in cosmology we call the critical density if the average matter density is sufficient to just slow down the Hubble expansion after infinite time to zero speed if we call that the critical density
22:20
then if we have that density it is possible to create galaxies in the time available because this dark matter that we don't see can be decoupled as we say from the radiation if this dark matter is in the form of exotic particles or other structures left over
22:41
from this original inflation phase of the universe then the gravitational concentrations which form into galaxies may be the result of perturbations of density of that dark matter which causes potential wells into which the neutrons and protons which make up our own galaxies actually fall and this then becomes a possible theory
23:02
so we would like this dark matter to be present and theoreticians have worked out what the universe might look like in the microwave background radiation at this time of about 10 to the 5 years after creation when hydrogen atoms are first possible
23:20
and this is the kind of structure one should see it depends of course on a lot of theory and different theoreticians will have different ideas this model is a model of Bond and Eustathio at Cambridge but you see that in the microwave background one should see fluctuations of brightness they're very small
23:41
they're on a scale of a few micro kelvin this is a very much elaborated picture which shows them rather enhanced but you see in a 10 degree by 10 degree square of sky one might expect to observe something like that if there is dark matter causing galaxy formation well that is but one
24:01
theory there are many others we're thinking of that particular theory then it's interesting to see that already observations of the microwave background are becoming important this is the same picture as you saw before but a little more scientific perhaps in the form of a graph we plot the temperature of the thermal fluctuations
24:22
that one would expect to see in the microwave background as a function of the angular scale and the thermal fluctuations first of all rise as the angular scale increases and it then flattens off this is simply because of local horizon effects which I can't tell you about
24:41
at the moment but the predicted fluctuations for different models are shown here and there's an observational point there famous observations by Euston and Wilkinson and already it is seen that we can rule out some cosmological models by direct observation and I think this is a very important point
25:00
models with parameters which come above this point here are clearly impossible well now this is not the only way of causing galaxy formation theorists are very clever people they're always coming up with strange ideas and one of the stranger ideas relating to grand unified theories and inflation is the possibility
25:20
that the universe is filled with well maybe not filled with but has within it cosmic strings these are vortices a vortex structure little enclosures of space time in the form of strings which could stretch across the whole visible universe or be in loops and they are strings of false vacuum
25:42
which although they are essentially quite massive exert no simple gravitational attraction but in motion they cause gravitational perturbations which could stir up the medium in space to form condensations which could later become galaxies cosmic strings
26:00
behave in a peculiar way and they distort space time in a complex fashion and I can only illustrate some results of computations here supposing there is such a cosmic string in space which has some shape like that then theoreticians tell us that it is in such a high
26:20
state of tension that if the string oscillates then the speed of the oscillations will be close to the velocity of light itself so that we can imagine this loop lashing about with a transverse motion equal to c under those conditions it produces a differential red shift across it well in fact it's a blue shift the space time is changed
26:42
in such a way that from one side of the string to the other when it is moving transversely one should observe a blue shift this will affect the microwave background radiation and computations show that if you had such a string in the universe its microwave background radiation would fluctuate as shown in that bottom picture you see the characteristic change
27:02
in radiation level as one goes from one side of that string to the other now radio telescopes should be able to see things like this if we actually design the right instruments that is going to be a problem because once again these differential temperatures are only of the order of a few millionths of a degree Kelvin
27:22
which is very difficult indeed to measure nevertheless I believe such things should be measurable and as you can see from the last two pictures the different theories now we can begin to sort out with good observations we can look at the angular structure of this radiation we can see whether strings exist or not and there is therefore a very
27:42
powerful influence could be exerted on the theories of course the observations are by no means easy and just to mention very briefly what some of the observation difficulties would be the radiation comes from a red shift of about Z equals a thousand to the earth it passes through clusters of
28:02
galaxies in which hot electrons will scatter the background photons by the inverse Compton effect and you will get apparent small absorption effects which have already been detected in the microwave background if the radiation passes through clusters containing hot gas which we know to exist near a home, well of course there are also a
28:22
background of faint radio galaxies which will confuse the structure we are trying to observe in the background nearer the radiation passes through our own galaxy there are components of radiation which have to be corrected for that's not impossible, finally the radiation passes through the troposphere for radio telescopes on the ground radiation from clouds
28:41
will also confuse these measurements and finally there is radiation from the ground itself which is of course around 290 degrees Kelvin which is a lot larger than 1 millionth of a degree Kelvin and that is what you are trying to measure so how is this going to work in practice here is the challenge speaking now slightly more technically
29:01
anybody who is seeking faint signals in the presence of noise knows that there is a fundamental relationship which relates to the square root of the bandwidth of your receiving system and the time you are prepared to take over the observations if you write down today what would be reasonable parameters for a radio telescope which could
29:21
detect 1 micro Kelvin against the microwave background radiation the receivers have a certain noise fluctuation themselves which tends to go down as you pay more money for the receivers but will not be much less at the moment than 50 degrees Kelvin if one uses a bandwidth which is about as large as you can get at the right frequency which is
29:41
500 megahertz and if the whole thing is at a frequency of about 15 gigahertz then if you observe continuously your sky for 2 months you will begin to see the fluctuations required well that's not impossible a PhD normally takes several years to achieve so that you have several observing times to see
30:01
but there is another difficulty and this is where I shall be near the end of my talk what would such a radio telescope actually look like? Well you see the structures you are trying to find are very large in relation to most radio telescope parameters the radio astronomers have been fighting to get high angular resolution now for 30 years
30:21
all of a sudden the requirements are quite different you need a radio telescope which is extremely sensitive to structures of the order of minutes of arc to a degree or so as I showed you from the pictures of cosmic strings and the predictions of the inflation theory on fluctuations what would that telescope look like? Well it will be a cluster of instruments like this
30:42
and here is the astronomer to the same scale it's going to look very like conventional radio telescopes but it's much smaller and now the largest mapping telescope we know is the VLA in the United States the very large array we would call this the VSA the very small array this is what it would look like
31:00
well we believe that with observations of this sort it will be possible ultimately to measure fluctuations in the microwave background these will be of fundamental importance indeed they may be the only way of testing grand unified theory there is an intimate connection between the early phases of the universe and theories of particle physics
31:22
grand unified theories and it could be that cosmological observations of this kind are the only way to check the theory they are immensely important they present a great challenge observationally and this is the kind of challenge which should be taken up by young radio astronomers today thank you very much