Gresham Lecture, Wednesday 15 December 2010
Unsolved Mysteries of the Universe
Professor Ian Morison
There is a, now famous, quotation from the American Secretary of Defence, Donald Rumsfeld, in which he says (put in a slightly clearer way):
There are known "knowns." These are things we know that we know.
There are also known unknowns - that is to say there are things that we now know we don't know.
But there are also unknown unknowns. These are things we do not know we don't know.”
Though this sounds like a riddle, the quotation actually encapsulates some real truths that are certainly applicable to astronomy. In the Gresham lectures I have spent my time telling you about the things that we know and that we are pretty certain about – the “known knowns” and I have certainly implied that there are things that, at the present time, we have no inkling of at all – the “unknown unknowns”. But we also have “known unknowns” – things that we know must exist or happen to allow our universe to exist but of which we have no real understanding. In this lecture we will look at some of the, as yet unsolved, mysteries of the Universe and describe the scientific instruments that are now coming into use or are in the planning stage that will help to unravel them.
Matter, Anti-matter and Dark Matter (1): what gives matter its mass?
This is one of the key questions in Physics today and vast sums of money are being spent in trying to answer it! In the 1960’s, Peter Higgs and others postulated that, very shortly after the origin of the universe, a “field”, called the Higgs field came into existence that now permeates the whole of the Universe. It is believed that it is this field that gives mass to the particles that pass through it. It is not easy to explain how this works but an analogy may help. Imagine a crowded, post Oscar’s party. If you or I could somehow be present, no one would take any notice of us and we could move around pretty easily - as a rather insignificant being (in this context) we have little “mass”. Now the young lady who has just been awarded the “best actress” Oscar arrives. Everyone will congregate around her, congratulate her and want to shake her hand. Her passage through the room will be very slow - she has a lot of “mass”! The throng of people in the room simulate the Higgs field and slow down those trying to pass through it dependant on (in this case) their perceived importance. It is the interaction of the Higgs field and the fundamental particles that give them their perceived mass.
We all know about “electric” or “magnetic” fields which are produced by charged particles. It is thought that such a field arises due to the existence of what are called “Virtual Particles” and the strength of the field is simply a measure of the density of these particles. What do we mean by a virtual particle? They are particles that have no long term existence but come into being and can only exist for a very short time before disappearing again. The typical time of their existence is determined by the Heisenberg Uncertainty Principle (one of the fundamental tenants of Quantum Theory) which, in one formulation, states that the heavier the virtual particles that spring into existence the shorter the time for which they can exist. The particle that relates to the electric and magnetic fields is the photon. So within a space permeated by an electric of magnetic field there will be virtual photons and the “force” that, for example, attracts unlike poles or repels like poles is due to the exchange of these virtual photons! In the same way, we believe that the strong nuclear force between the quarks that make up neutrons and protons is the result of interaction of virtual gluons and the force that holds neutrons and protons together in nuclei is due to the exchange of virtual mesons such as the pi and rho mesons. So, one way of looking at the interaction of particles and the Higgs Field is the exchange of the virtual particle that is associated with it which, in the case of the Higgs field, is called the Higgs Boson.
If we could prove the existence of a Higgs Boson then the theory would be proven. But, remember, these particles are virtual particles - they do not hang around to be detected! However there is a trick that can, in principle, be played. A virtual particle has “borrowed” energy to come into existence and must almost instantaneously pay it back. However, if you could somehow give it sufficient energy during the brief moment of its existence it could become real. There is a real problem here. Though we do not know the energy that is required we do know that it is very high and, to date, have not been able to create sufficient energy in a sufficiently small volume of space to create a real Higgs Boson. Even then, it would decay almost instantaneously into a myriad of other particles - decay products - but it is hoped that precise analysis of these could prove that a Higgs Boson had had a brief moment of existence!
This is why a key objective of the Large Hadron Collider is to attempt to detect the Higgs Boson – regarded as one of the holy grails of Particle Physics and a necessary component of the Standard Model that attempts to describe the fundamental particles that make up our universe.
The Large Hadron Collider (LHC) is the world's largest and highest-energy particle accelerator and lies in a tunnel 27 kilometres in circumference beneath the Franco-Swiss border near Geneva, Switzerland. It is designed to collide opposing particle beams of either protons at an energy of 7 teraelectronvolts (TeV) per particle, or lead nuclei at an energy of 574 TeV per nucleus. The proton collisions are those that will be used in the search for the Higgs Boson whilst the lead nuclei collisions (that began in November 2010) have a bearing on the matter-antimatter problem that will be discussed later.
The collider tunnel contains two parallel beam pipes, each containing proton beams which travel in opposite directions around the ring. Over 1,600 superconducting magnets keep the beams focussed on their circular paths, which cross at four points where the two beams will interact. In total, 96 tonnes of liquid helium are used to keep the magnets at their operating temperature of 1.9 K (−271.25 °C), making the LHC the largest cryogenic facility in the world at liquid helium temperature! When operating at full power the protons will have an energy of 7 TeV and circulate though the rings at ~0.999999991 c, about 3 m/s slower than the speed of light and taking less than 90 microseconds to make one revolution. The protons are bunched together so interactions take place at discrete intervals of time so that the electronics in the detectors that are placed at the intersecting beam points can complete their measurements.
The detectors that will be used to search for the signature of the Higgs Boson are the ATLAS and CMS detectors. ATLAS is about 45 meters long, more than 25 meters high, and weighs about 7,000 tons. It is about half as big as the Notre Dame Cathedral in Paris and weighs the same as the Eiffel Tower! When running it produces a prodigious amount of data which, if all were recorded, would fill 100,000 CDs per second. In fact, very high speed computers analyse the data in real time and only record data when it looks as though an interesting event has occurred - a rate equivalent to 27 CDs per minute. The CMS detector is built around a huge solenoid magnet. This takes the form of a cylindrical coil of superconducting cable that generates a magnetic field of 4 teslas, about 100,000 times that of the Earth. The magnetic field is confined by a steel 'yoke' that forms the bulk of the detector's weight of 12 500 tonnes.
It has been said that the search for the Higgs Boson is not like looking for a needle in a haystack but one in 10,000 haystacks! It is thought that it could take several years before sufficient events to have taken place to prove, or otherwise, its existence.
Matter, Anti-matter and Dark Matter (2): Why do we have a Matter Universe?
In 1928 Paul Dirac took an important step towards bringing quantum physics into conformity with Einstein's special theory of relativity by devising an equation (now called the Dirac equation) that could describe the behaviour of electrons. This equation provided a natural explanation of one of the electron's intrinsic properties - its spin. [It is the change in the spin of an electron with respect to a proton in a hydrogen atom that gives rise to the 21cm hydrogen line that will play a key part in a later part of this lecture.]
Considering how his equation could be interpreted, in 1931 Dirac proposed that there should exist an 'anti-electron' - a particle with the same mass and spin as the electron but with the opposite electrical charge. By predicting the existence of this antiparticle, now called a positron, Dirac became recognized as the 'discoverer' of antimatter - one of the most important discoveries of the last century. In 1932, Carl Anderson, an American professor, was studying the tracks left by showers of cosmic ray particles in a Wilson cloud chamber. He saw a track left by "something positively charged, and with the same mass as an electron." Anderson had detected a positron and so proved that Dirac's prediction about antimatter was accurate.
Dirac asserted that every particle has an "antiparticle" with nearly identical properties, except for an opposite electric charge. And, just as protons, neutrons, and electrons combine to form atoms and matter, antiprotons, antineutrons, and antielectrons can combine to form antiatoms and antimatter. Worrying why our universe seems to be composed of matter only, his theory led him to speculate that there may even be mirror galaxies or universes made entirely of antimatter, but no experiments have yet been able to detect the antigalaxies or vast stretches of antimatter in space that Dirac imagined.
Should matter come into contact with antimatter (they are attracted to each other as they have opposite charges) it was discovered that they would instantly annihilate each other giving rise to a burst of radiation. This is the most efficient conversion of mass to energy possible and this is, no doubt, why it was used to power the Starship Enterprise and so allow it to travel at “warp” speeds.
The question that really confounds physicists today is why our universe appears to be made of matter only. The standard theories of physics say that when the universe came into existence some fifteen billion years ago in the Big Bang, the energy created must have formed equal amounts of matter and antimatter - the laws of nature require that matter and antimatter be created in pairs. But within less than a second of the Big Bang, matter particles somehow outnumbered antimatter particles by a tiny fraction, so that for, say, every billion antiparticles, there were a billion and one particles. All the antiparticles were annihilated giving rise to the radiation that makes up the Cosmic Microwave Background (then in the form of gamma rays). So, within a second of the creation of the universe, all the antimatter was destroyed, leaving behind only matter. So far, physicists have not been able to identify the exact mechanism that would produce this apparent "asymmetry", or difference, between matter and antimatter to explain why there arose this tiny excess of matter over antimatter.
Today, antimatter is created primarily by cosmic rays - extraterrestrial high-energy particles that form new particles as they penetrate the earth's atmosphere. It can also be produced in accelerators like that at CERN, where scientists create high-energy collisions to produce particles and their antiparticles. In one of the most recent CERN experiments, published in November 2010, a team has succeeded in producing a significant number of atoms of anti-hydrogen – made up of an antiproton and a positron: antiprotons at very low temperatures and thus almost stationary were first compressed into a matchstick-sized cloud 20 millimetres long and 1.4 millimetres in diameter and then a similar cold cloud of positrons were added giving rise to around 38 anti-hydrogen atoms which were successfully trapped inside a magnetic bottle for one sixth of a second. This is, for the first time, allowing scientists to study antimatter in detail and so may help to determine why matter appears to be in the ascendant.
Physicists believe that there must be a subtle difference in the way matter and antimatter interact with the forces of nature to account for a universe that prefers matter, but have not, as yet, been able to confirm this experimentally. In 1967, the Russian theoretical physicist Andrei Sakharov postulated several complex conditions necessary for the prevalence of matter. One of these is a "charge-parity" violation, which is an example of a kind of asymmetry between particles and their antiparticles in the way that they decay. The goal of the CERN scientists is to find evidence of that asymmetry.
The LHC Beauty (LHCb) detector is designed to answer a specific question: where did all the anti-matter go? In order to do this, the LHCb is investigating the slight differences between matter and antimatter by studying a type of particle called the "beauty quark". Although absent from the Universe today, these were common in the aftermath of the Big Bang, and will be generated in their billions by the LHC, along with their antimatter counterparts, anti-beauty quarks. 'B' and 'anti-b' quarks are unstable and short-lived, decaying rapidly into a range of other particles. Physicists believe that by comparing these decays, they may be able to gain useful clues as to why nature prefers matter over antimatter.
The LHC produces many different types of quark when the particle beams collide. In order to catch the beauty quarks, LHCb has developed sophisticated movable tracking detectors close to the path of the beams circling in the LHC. In September 2010, the LHCb observed what are called “beautiful atoms”. The atoms are bound states of the beauty quark and anti-beauty quark. [Quarks can only exist in triplets, such as in Protons (Up, Up, Down) and Neutrons (Up, Down, Down), or as quark-antiquark pairs.] The beautiful atom is 10 times heavier than the proton and has a size slightly smaller than the size of the proton.
A further approach to the understanding of what happened during the Big Bang would be to try to recreate the conditions that then existed. Since November 2010 the LHC has been used to collide lead ions, 208 times heavier than a proton and the first results have shown for the first time that the proton and neutrons that make up the nuclei can “melt” into their constituent quarks and gluons and so form what was thought be the primordial “soup” that existed around a billionth of a second after the Big Bang.
Matter, Anti-matter and Dark Matter (3): What is Dark Matter?
One of the biggest mysteries in modern astronomy is the fact that over 90% of the Universe is invisible. This mysterious missing stuff is known as 'dark matter'. The problem arose when astronomers tried to weigh galaxies. There are two methods of doing this. Firstly, we can tell how much a galaxy weighs just by looking at how bright it is and then converting this into mass using what is called the mass-luminosity relation. The second way is to look at the way stars move in their orbits around the centre of the galaxy. In just the same way that we can calculate the mass of the Sun by knowing how far we are from it and the speed at which we rotate round it we can calculate the mass of a galaxy by studying how fast stars at the very edge move around it. The faster the galaxy rotates, the more mass there is inside it. But when astronomers such as Jan Oort and Fritz Zwicky did these calculations in the early 1930’s the two answers didn't match. As they were very confident that both methods were sound they came to a startling conclusion - there must be a form of matter out there that we cannot see - which became known as 'dark matter'.
So what is dark matter made of? No one knows for sure. Normal matter, making up the stars, planets and ourselves, is made of atoms, which are composed of protons, neutrons and electrons. Scientists call this "baryonic" matter. A small part of the dark matter is of the normal, baryonic variety, including brown dwarf stars, dust clouds and other objects such as black holes that are simply too small, or too dim, to be seen from great distances.
The amount of ordinary or baryonic matter in the Universe, whether visible or dark, can be estimated on the basis of the relative quantity of deuterium and helium that was formed three minutes after the Big Bang. If there was a lot of baryonic matter at that time, collisions first between nucleons and later between nuclei would have been very probable and the percentage of deuterium should now be very small because deuterium nuclei give rise to helium; if instead there was little baryonic matter, the quantity of deuterium should be relatively greater. From the most recent measurements of the current quantities of deuterium and helium, it can be deduced that the baryonic matter present in the Universe is only about one seventh of that needed to keep stars in their galaxies and galaxies in their clusters.
We know that a component of dark matter is in the form of neutrinos which do not have an electric charge and rarely interact with ordinary matter. The estimated mass for neutrinos is very small and if we multiply it by the huge number of neutrinos present in the Universe, we obtain a contribution to the total mass of the Universe which is slightly less than that from visible matter. As they are moving at the speed of light they form what is called “hot dark matter”. If there were too much hot dark matter it would be very difficult for galaxies to form and we would not be here, so this is a comforting, if not surprising, result!
Thus we believe that the major contribution to dark matter is in the form of slowly moving (by comparison) massive particles called “cold dark matter”. Theoretical physicists have come up with various hypotheses as to what these mysterious particles could be. Many of these come out of the physics theory called “supersymmetry”. A good candidate is the neutralino which is the supersymmetric electrically neutral particle which has the lowest predicted mass.
Another possibility could come from nuclearites, combinations of up, down and strange quarks. These would have a higher density than ordinary nuclei and would be stable, even for masses much greater than those of the uranium nuclei. A key goal of the LHC is to attempt to produce such particles. So far, high energy accelerators have not observed any, which implies that they are very massive. As the LHC will be able to produce energies that are far higher than any existing accelerator (and hence be able to create more massive particles) there is hope that dark matter particles might be detected. It is thought that its detection might be quite an early result from the ATLAS and CMS detectors and, even if no detections were made, the LHC results would, at least, eliminate some of the possibilities.
If we know very little about dark matter, we know even less about dark energy! It gives rise, we believe, to a repulsive force that is arises out of the vacuum of space. In fact, Einstein, postulated it in the set of equations derived from his General theory of Relativity that he used to try to explain a static universe – the repulsive force just balancing the attractive force of gravity. He gave it the name of the Cosmological Constant, . When Hubble found that the Universe was expanding he realised that he had made the biggest blunder of his life and, if he had not brought in the repulsive force, he could have predicted that the Universe was expanding. However, observations of very distant Type 1a supernovae made in the late 1990’s indicted that, in complete contrast to the then current ideas of cosmology which predicted that the rate of expansion would be slowing down, the universe was, in fact, expanding at an ever increasing rate - the switch from a slowing expansion rate to an expanding one happening some 4-5 billion years ago. One explanation is that it is the cosmological constant - a constant energy density uniformly filling space - that is the cause. In the current standard model of cosmology - the Lambda-CDM model - dark energy currently accounts for 74% of the total mass-energy of the universe. [CDM is Cold Dark Matter.]
There are, however, other possible scenarios apart from the term which include scalar fields such as quintessence (fifth force) whose energy density can vary in time and space. These can be difficult to distinguish from a cosmological constant effect because the changes may be extremely slow. Very high-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time. In general relativity, the evolution of the expansion rate is determined by the cosmological equation of state which is the relationship between temperature, pressure, and the combined matter, energy, and vacuum energy density for any region of space. Measuring the equation of state of dark energy is one of the biggest efforts in observational cosmology today.
There are two new planned optical instruments that will help to distinguish between the various possibilities by providing the high precision observations that will be required.
The European Extremely Large Telescope (E-ELT)
A major role of JWST is to reveal the story of the formation of the first stars and galaxies in the Universe. Theory predicts that the first stars were 30 to 300 times as massive as our Sun and millions of times as bright, burning for only a few million years before exploding as supernovae. The emergence of these first stars marked the end of the "Dark Ages" in cosmic history and an understanding these first sources is critical, since they greatly influenced the formation of later objects such as galaxies. When the first massive stars exploded as supernovae their remnants would have been black holes. These, initially small, black holes started to swallow gas and other stars and merged to become the huge black holes that are now found at the centres of massive galaxies. It is hoped that the JWST will shed light on the nature of the relationship between the black holes and the galaxies that host them.
Even now, astronomers do not really know how the galaxies formed, what gives them their shapes and what happens when small and large galaxies collide or join together. The JWST will hopefully answer these questions. By studying some of the earliest galaxies and comparing them to today’s galaxies we may be able to understand their growth and evolution. It will also allow scientists to gather data on the types of stars that existed in these very early galaxies. Follow-up observations using spectroscopy of hundreds or thousands of galaxies will help us understand how elements heavier than hydrogen were formed and built up as galaxy formation proceeded through the ages. These studies will also reveal details about merging galaxies and shed light on the process of galaxy formation itself.
How do Stars and Planets Form?
Stars and their planetary systems form together in thick dust clouds which makes it almost impossible to study their formation at visible wavelengths. Almost all of the obscuring gas and dust seen in visible images views may entirely disappear when viewed in the infrared, so that the proto-stars lying behind the gas and dust become easier to see. So infrared astronomy can penetrate dusty regions of space such as molecular clouds and observe the young stars forming within. As indicated above, infrared observations are also key to observing young stars in the early Universe as the visible light radiated by them will have been shifted down to infrared wavelengths by the expansion of the Universe that has occurred since the light was emitted.
The James Webb Space Telescope with its excellent imaging and spectroscopic capabilities will allow us to study the formation of stars and will also be able to image the proto-planetary disks around stars helping us to understand how planets may form in orbit around them. These studies will also be greatly helped by the high resolution imaging in the wavelength bands just beyond the infrared - the sub-millimetre and millimetre parts of the electromagnetic spectrum – that will be achieved using …….
The Atacama Large sub-Millimetre Array (ALMA)