Week 10: Lecture 47: A non-specialist’s introduction to Astrophysics

So, we now change gears a bit from neutrino physics, we now come to astrophysics. And I should admit that uh I mean, this is a non-specialist introduction to astrophysics. I'm not a astrophysicist myself. So, this is a is my view of astro astronomy and astrophysics. These are the references. Uh four main references, but of course, there are other you know, there are review articles in Nature, Science, Review of Modern Physics, Wikipedia even. And then, their individual big uh observatories, they have their own websites. Square Kilometer Array, SKA, has its website. Similarly, LIGO and IceCube, etc. So, astrophysics uh actually started out with astronomy. Astronomy is a I mean, it's a Greek word that deals with science that study the laws of stars, indeed study of everything in the universe outside of the Earth. Now, astrophysics uses physics to study these astronomical objects and phenomena. So, astronomy is basically observational. Uh Astrophysics uses physics uh and perhaps even chemistry when you want to talk about compounds that are formed and so on, uh to study astronomical objects and phenomena. Cosmology is a sort of wider thing in the sense that it studies the origin and the evolution of the universe, large-scale structures in the universe. Uh nuclear astrophysics is part of astrophysics because it studies the processes that are relevant to stars and uh other cataclysmic events. So it looks at for instance nuclear reactions, beta decays, photo nuclear processes, and so on. How do we study the universe? How How in astronomy how do we look at the universe? What probes do we have? So of course the most powerful probe is that of electromagnetic radiation. So what is shown here is the atmospheric opacity as a function of the wavelength. It ranges right from fractions of a nanometer to wavelengths of the order of kilometers and more. And as you can see the opacity is very high, close to 100% for things up to a few hundred nanometers. So this falls in the visible region, so you can see purple, green, yellow, and red. And this is what our eyes also sensitive to. But then the the opacity is not really 100% for further range of wavelengths, right up to the far infrared, okay? So up to about wavelengths of the order of 50 or 30 micrometers. Then again, beyond about uh few millimeters uh then it again drops. And there is a big window where the opacity is almost zero from a few centimeters uh wavelength to uh of the order of 10 or 15 meter wavelength. So this is a huge window. So this is the radio astronomy region, okay? This is This is how radio telescopes were built. And this is in the optical region, okay? Telescopes in the optical region. So of course since the opacity is high here for atmosphere, then you have to go outside of the atmosphere. So you you have to put these uh telescopes outside of the atmosphere in satellites. And then of course you can look at gamma rays, x-rays, UV light, etc. that is blocked by this atmosphere. Similarly, the visible of course you can do detection on the earth. You have telescopes on the earth, very big ones at that. And the infrared since again it is absorbed you have to go outside, you have to go to telescopes in satellites. In fact there is Hubble telescope was is the most famous one for a long time. Now of course we have the James Webb telescope as well which is operating at a much higher distance from the earth. And which mainly has this infrared telescope which is doing lots of detections of very far >> [snorts] >> stars and galaxies. Radio waves of course because of the atmospheric transparency to radio waves of course we can build big telescopes on the ground. This is an advantage because when you carry something in space it always costs much more than where you would build it on the ground. Then of course very long wavelength radio waves are blocked. So again if you were to nobody has built for very long waves but if you were to do that you again you would have to go in a satellite. Okay. Now let's look at the in general what is the angular resolution of your device of your telescope dependent on? Well, it is angular resolution, maximum angular resolution, is of course proportional to lambda and inversely proportional to the aperture. So if you have a larger aperture you have better angular resolution. If you have a smaller wavelength you have a better angular resolution. Of course if you have a poorer lambda if you have lambda, then you require a higher aperture. Okay. So, the human eye, for example, in comparison, has an angular resolution of the order of about point you know, .01 of a degree. Okay. Uh this is for a wavelength of about 500 nm, assuming an aperture of about 4 mm. The intensity, of course, which it can detect is really low. It can detect Basically, it has single photon detection capability. And this is the reference here. So, sorry. Put it here. Yeah. The uh 30-m telescope, on the other hand, by definition, is 30 m wide. So, it has an angular resolution which is much better. It's about 1.7 * 10 ^ -8 of a radian, which means 3.5 * 10 ^ -3 arc second. The angular diameter of the Sun and Moon, which we see as quite big objects, are both about point .53°. They subtend .5 degrees. Okay. So, this And they are very similar, and that is what makes uh eclipse possible, right? Lunar solar eclipse. Uh So, if you want to image a black hole, on the other hand, let's say at the center of the Milky Way galaxy, it is quite distant. Uh so, there you need to go to short wavelengths. Because it is distant. So, the angular width of that is small. So, you need a lambda which is small, and of course, the A, the aperture that is needed it comes out to be about 10 to the 4 km. How can you do that? You can put uh you know, these radio telescopes, much smaller dishes than 10 to the 4 km. They'll be of the order of tens of meters. But you put them at various places on the Earth. And then you uh measure the phase relationships between the signals that you get and by doing a very careful job you can actually get a picture of the black hole which is so far away. In spite of the fact that the wavelength is large as compared to light for instance or to x-ray or to gamma ray and so on. So this was done by the Event Horizon Telescope collaboration that's given here and uh already in 2019 they published the picture of that but the full paper came out in 2024 in this journal Astronomy and Astrophysics. Uh and uh so I should have made it darker here. Okay, so this is done for the M87 uh you know galaxy which is relatively close to the earth 55 million light years away. Uh the big black hole which is billions of times the mass of the sun and this was as I said imaged recently by this Event Horizon Telescope. This is a picture of that. Now why this is so is one can think about it. This is this arises this is not actually this is converted from the radio frequency image to a color image so that's why you see and then these are more intense parts presumably there is matter which is uh you know traveling towards us so it's Doppler shifted uh to smaller wavelengths and the matter here is going away and so this is as a result of a spinning black hole. Okay. The radio telescopes there are many of them. The earliest big ones was the Jodrell Bank in UK. Then we have of course our own array of meter wave telescopes which is the So, it's called the giant meter telescope GMRT radio radio telescope it should be. Uh so, this called the GMRT telescope. Uh that is it uh near Pune and there's also a new array coming up, which is has a worldwide collaboration. Lots of laboratories are contributing to this. Is the so-called square kilometer array in Australia and in South Africa at two places. So, they have different kind of detector in either of these places, but it's supposed to cover something like square kilometer the the total dish area. Okay. The uh ultra high uh energy gamma ray detector arrays are also there, which are used Cherenkov photons. We have indeed one such called MASS, but there are others elsewhere in the uh on the globe. Then, we have space-borne telescopes such as the Hubble and now this is no longer upcoming. This is uh this is already there. Um and it's producing great pictures of as I said of the universe in both the infrared and in the visible region. Uh We have We have other probes such as cosmic rays, charged particles like protons, electrons, and so on. Uh the Pierre Auger Observatory in South America is a big one. And then, there is also a space-based one called the AMS-02 in the International Space Station. Uh that has also produced some, you know, intriguing data. Uh very accurate data on charged particles and so on. Uh then, we have neutrino detectors such as Super-Kamiokande, IceCube, and KM3NeT. This is again a water-based detector uh photomultiplier sunk in water in the Mediterranean Sea. So, that this is a mainly European collaboration. And then finally, we have gravitational waves which have been detected by LIGO in Washington Hanford. And uh So, Washington and Hanford, these both of these uh arms are two-arm based gravitational wave detectors detected a huge event of a two black holes merging together in 2016, about 100 years after gravitational waves were predicted by from the Einstein equations of gravitation. And then of course, now there are other detectors in Italy and Japan. There is one upcoming in India in Hingoli. It's supposed to come up by 2030 in another 4 years' time. And there's also R&D which is based on space-based laser interferometer interferometer where you can get much larger distances uh of the order of hundreds of kilometers uh or even tens of thousands of kilometers. And so, that will make it much more sensitive to different domain of uh frequency which of these gravitational waves. So, this is a nice picture of the GMRT facility that we have in India, which is as I already said in Khodad near Pune in India. And uh The next one shows the LIGO gravitational wave detector at Hanford. Uh so, this is on the southern part of the US, south- eastern part of US. The Washington one is the northwest uh part of the US. And so, there are these two 4-km arms uh which which are based on a laser interferometer. So, the laser beam is split, goes this way, comes back, and in fact, it goes hundreds or thousands of times back and forth so as to amplify this small this tiny deformation in the in the length that you have, tiny strain of the order of 10 to the minus 21 or so. And uh this is the other arm. So, by doing this uh interferometry, which is basically of the Michelson-Morley type, but not this time using lasers and a very large uh you know path through which it travels, they have been able to measure these things. And uh so, as I said, first such event was detected in 2016. So, what have we learnt about the uh universe? The sun is a typical star on the edge of the Milky Way, okay? The Milky Way is part of a cluster of galaxies, of which there are many in the universe. There is a overall expansion of the universe. This was uh discovered by Hubble in 1929, and stars which are farther away move faster, okay? So, the velocity at which they go out uh they're going out is proportional to the distance. Uh and that proportionality constant is the Hubble so-called Hubble constant. Uh There is electromagnetic radiation background in all directions. This is another finding. Uh so-called CMBR, and uh of course, this uh is corresponds to the best black body spectrum that we know, and it has a black body spectrum of a body with temperature 2.7 Kelvin. Of course, there are more decimal places in this. I've said it's uh approximately of this order, just to give you an idea. Uh so, this is the cosmic microwave background radiation that we have seen. Our place in the universe, okay? So, if you go from the universe, you have a local supercluster, then a local group, then the Milky Way galaxy is part of this local group, then you have the solar system, and then of course you have the earth. So this is the sun with all its planets, the biggest planet being Jupiter, and then the earth is third planet closer closest to the sun, and this is the blue planet, a small speck in the whole universe that we live in. Some numbers just to give you some ideas. The radius of the sun is about 0.7 million kilometers. The mass of the sun is of the order of a couple times 10 to the 30 kilograms. Earth is about a million times smaller in mass. The luminosity is about 4 10 to the 26 watts. Uh and one astronomical unit, which is the mean distance between the earth and the sun, is about 1.5 10 to the 8 kilometers. A parsec is uh is one atomic unit per arc second. Uh so that's about uh three light years. So if you have one atomic astronomical, sorry, not atomic, sorry, astronomical unit uh subtending one arc second, then that distance corresponds to about 3.3 light years. That's about 3 10 to the 13 kilometers. The Hubble constant is about 70 kilometers per second per megaparsec. Uh of course there is as of now there is a slight tension between two kinds of measurements of the Hubble constant. They don't seem to agree within their error bars. Uh so that's a but that's approximately this is true 7 it's of the order of 70 km per second per megaparsec. The radius of our Milky Way is about 25,000 light years. The number of stars in the Milky Way is about 10 to the 11. And the mass of this Milky Way is about 10 to the power of 12 times, that's about a trillion times the mass of the sun. We are part of the local cluster of about 30 galaxies within about a megaparsec. The nearest galaxy cluster is Virgo, which is about 14 megaparsec. And uh about 10 to the three uh galaxies uh in this cluster including M87. A cluster of clusters is called a supercluster is on the scale of 30 to 300 megaparsecs. Some more constants given here, and uh anyway, I won't read them out. These are good for back of the envelope calculations. Okay. So, you start out with the gas cloud which is formed, and then uh you have uh matter accreting you can form a young star, and then later it evolves in time you can get a white dwarf which is about less than eight times the solar mass, or you can have a supernova, which then blows up and you can get a neutron star. Uh so, this happens when the mass of this uh star is eight to 20 times the mass of the sun. And if it is greater than 20, then of course you you form a black hole. Of course, then in that case, uh since it's an explosive event, you have matter coming out, but you also have gravitational radiation if it is non uh isotropic non-spherical situation. Uh supernova are sites for rapid neutron capture leading to elements beyond iron, and uh so, up to iron you can go through the fusion reactions. Beyond iron you have to go through uh neutron capture, and this is one of the one of the places where it can happen, a supernova, where uh electron proton combines to give neutrons, so then the neutrons are very uh abundant in the in this kind of explosion, and you can get heavy elements. These heavy elements are then dispersed in the cosmos to be accumulated in next generation stars, and you can also have neutron star mergers, and in fact, and they they can also be places where you synthesize the heaviest elements like uranium and so on. In fact, it is believed that such mergers can also lead to superheavy elements. Uh however, we don't have experimental evidence for that, but in principle, you can get uh you know, things that can be longer lifetimes, but of course, on a uh on a scale of 10 billion years, they won't survive uh likely. Uh but they can be formed uh and maybe they they're there for a billion years after they're formed. Uh some of the doubly magic uh superheavy elements. For the heaviest stars, even the neutron degeneracy pressure cannot withstand the gravitational force, and this ultimately leads to further contraction into a black hole. Spinning black holes, of course, acquire accretion disks, a magnetic field, and are likely source they're called blazars of the most energetic cosmic rays, which we have seen, by the way, in uh experimental arrays in the Pierre Auger array and so on. This is the spectrum of the cosmic microwave background. And as I said, I mean, the error on this measurement is much less than the thickness of these lines. It is a kind of uh uh 100 parts per million accuracy that we know the spectrum to. You can see this is one part in uh 2 2,700. This is the kind of accuracy that you have on the temperature. It's one of the best blackbody spectra that we have measured. Present-day understanding is that the universe began with the Big Bang. It was coined in a sort of slightly derogatory way by Fred Hoyle, who was a proponent of the steady-state universe. And perhaps this happened through a quantum fluctuation. And as it turns out, the total uh potential energy, which is negative, uh due to gravitation of, uh you know, matter and radiation, is of the same order as the total, you know, mass and total energy that we see in the universe. Any proton, for instance, has a mass that's that's contributed to its energy. The photons also, and the other stuff that is there in the universe. And that roughly matches with the potential energy. So, total uh V plus E uh could be close to zero. So, this could arise, for instance, from a fluctuation, a quantum fluctuation. The initial singularity grew rapidly uh through something called inflation, which of course we also don't quite understand, but it moved very rapidly at evolving probably speeds greater than that of light. And then, you have energy density fluctuations, uh which led to clumps of matter that condense into stars, galaxies, and clusters of galaxies, and the indeed the large-scale structure of the universe. Initially, only elementary particles and antiparticles were there, like quarks and antiquarks, gluons, leptons, gamma rays, uh WZ bosons, and so on. And then, within a few seconds, you formed nucleons, uh and so this is neutrons and protons. And then, after a few minutes, you formed nuclei when the temperature was about 5 MeV. And then, atoms at about 380,000 years, when the temperature was about 10 electron volts, so that the hydrogen atom, of course, could survive. The electron binding energy of the 1s electron in hydrogen is, if you remember, about 13.6 electron volts. So, when the temperature dropped to below this, then of course atoms could form. And we know now we know that the present age of the universe is about 14 uh giga years, 14 billion years. So, the evolution of this universe is captured in this cartoon. This is from the Vigyan Samagam brochure, which is of course also based on earlier such cartoons. And if you have the Big Bang here, then this is the time evolution. You have, you know, detectors that could that sensitive to the early phase of this Big Bang. Uh early times after the Big Bang and so on. And this is the present-day universe, this is our solar system. And LIGO is even looking at events which are a few billion years They're looking at Big Bang I mean black hole mergers which have occurred a few billion years ago. And so on. So This is just a cartoon which shows the various phases and the detectors that we have or the phenomena that we would be sensitive to. So a recap on the time evolution of the universe. The first picosecond or so is called the I mean before that is called the Planck epoch where we can say the four interactions emerge. Uh otherwise maybe there is just a unified interaction. So gravitation, then electromagnetic, then the weak, and then the strong. And space expands rapidly initially and including fluctuations. In the microsecond you have a quark-gluon soup. In a second you have neutrinos which decouple leading to the cosmic neutrino background. Quarks coalesce to find to form nucleons. In 2 minutes the conditions are right for primordial nucleosynthesis or 25% of protons and all neutrons fuse to form deuterium and then to helium. Uh then you have in 20 minutes fusion which stops, but the hot plasma still does not sustain atoms because photons can ionize atoms which are formed. And in about 50 kilo years matter dominates photons. In about 100 kilo years you have the first molecule of helium hydride, helium H plus. And this was detected in 2019 in this particular nebula nebula using a terahertz spectrometer on an airplane. Uh in about 370 kilo years 370,000 years, you have neutral atoms which form and the universe is transparent to photons which decoupled decoupled from matter. Uh so initially it is pale orange in color. Later as the universe expanded, this Doppler shift gave rise to microwave background. So if you looked at the universe, if you had an eye which was sensitive to microwaves as well, you would only see it in the microwave and not in the So you would see of course spots which are stars which are colorful, which are maybe blue, which may be red, and so on. But the background would be like uh cosmic microwave background. Okay. So up to about a giga year, the thermal radiation becomes dark, it goes into infrared as the universe expands beyond 3 mega years. So 0.2 to 0.5 giga years, you get the earliest stars, galaxies form, there's reionization by high-energy gamma rays from stars, dwarf galaxies, etc. between 0.25 to 0.9 giga years. 1 giga year to 13.8 giga years, universe was like what we think it is today. Of course, this is a This is what we believe as of now. If there is new evidence which, you know, fractures some of these beliefs, that might happen. Now, will this expansion continue or will it even accelerate? We don't know yet. Uh there are some hints in which can be interpreted in either way, but for sure we don't really know. However, the universe is not made of only matter and radiation, we think. Uh our present-day understanding is that matter antimatter was produced in equal approximately equal quantities in the Big Bang, but uh there is there is CP violation, so this introduced small differences. There are more particles than anti-particles, and the particle antimatter is annihilated to produce radiation, but a small excess of the level of BBB or so of matter was left behind, and that is what is the present-day universe. So, this the conditions required for such a matter excess were they were worked out by Andrei Sakharov, a Soviet physicist, who is also known as the father of the hydrogen bomb in the USSR, then USSR. So, the Big Bang nucleosynthesis proceeds through these reactions, and since it proceeds very fast, it can only make light nuclei, such as seven beryllium lithium, seven lithium, and so on. And the standard Big Bang nucleosynthesis of primordial elements is shown here. So, on on one axis you have the temperature, and of course, it is in reverse. So, you have a high temperature initially, and then it cools, and this is the evolution in time, right from a fraction of a second to about million seconds, okay? So, this is like about uh one would say a few days. So, initially you have neutrino decoupling, then you have neutron proton decoupling, then you have nuclei which are formed here, and then they also freeze out. Okay, so you have a D to H ratio, three helium to H hydrogen ratio, tritium to hydrogen, seven beryllium to hydrogen, seven lithium to this, and so on, and six lithium to hydrogen ratio. So, this is taken from this reference in 2010. So, this is the reference. Okay, so in summary, I have given a very brief and perhaps sketchy introduction to astronomy and astrophysics. Uh What is our present-day understanding of the time evolution of the universe and also nuclear synthesis in the first 10 minutes after the Big Bang? Thank you. >> [music] [music]