Cosmology

1. Syllabus --------------------------------------------------------------------------------------------------------------------- Special theory of relativity. General theory of relativity. Friedmann universe models, Hubble constant, cosmological red shift Elementary particles and interactions. Modern cosmology, phases and processes near after the Big Bang. Evolution of the Universe with dark matter and dark energy. --------------------------------------------------------------------------------------------------------------------- Cosmology

2. The study of the Universe at large, its origins and evolution. It is distinct from cosmogony, which is the study of the origin and evolution of objects within the Universe, such as galaxies. Although the roots of cosmology can be traced back to ancient myths and legends, and the Greek study of the way the planets move, modern cosmology is essentially a mathematical description of the behaviour of spacetime on the largest scale, using equations derived from Albert Einstein’s general theory of relativity. So the date on which modern cosmology was born can be set very precisely, as 1917, the year Einstein first applied those equations to describe the Universe at large. Although some theorists have developed alternative theories of gravity and spacetime, which have led to cosmological models different from those derived from Einstein’s theory, these alternative cosmologies have now been ruled out by observations. Within the framework of Einstein’s theory, there have been two great cosmological hypotheses, the Big Bang and the Steady State. Both these models conform to the observational evidence that the Universe is expanding as spacetime stretches, but in the big bang model this is seen as evidence that the Universe was born out of singularity a finite time ago, while in the Steady State model it was required that new matter should be created continuously to fill the gap between the galaxies as they moved apart, so that the overall appearance of the Universe stayed the same. Cosmology

3. The simple Steady State hypothesis has been found to be incorrect, because there is now clear observational evidence that the Universe changes as time passes. This leaves a variety of possible Big Bang models, which can be considered as possible descriptions of the real Universe. To within the limits of observational accuracy, our Universe is indistinguishable from flat model which obeys the laws of Euclidean geometry. This is the simplest possible Universe allowed by Einstein’s equations. Cosmology

4. Newton’s mechanics Electromagnetic field theory Theory of the relativity Special theoryof relativity Gravitation General theory of relativity Relation between classical mechanics, electromagnetic field theory and theory of relativity

5. Special theory of relativity Specialtheory of relativity: Description of the relationship and interactions between moving object, developed by Albert Einstein early in the 20th century. The theory was first published in 1905, in a mathematical form based on equations; its implications can be more clearly visualized, however, using a geometrical description of events taking place In a fourdimensional spacetime, first applied in this context by Herman Minkowski in 1908. The “special” theory gets its name because it applies only to be the special case of objects moving at constant speeds in straight lines – that is, at constant velocities. It does not deal with accelerated motions, including the acceleration caused by gravity. The later extensions of Einstein’s theory to deal with gravity and other accelerations (the general theory of relativity) developed the geometrical model of spacetime further. The key features of the special theory are that, from the point of view of an observer who is regarded as stationary (in his or her own inertial frame), time recorded on a moving clock will run slow, a moving object will shrink in the direction of motion, and the moving object will gain mass. The speed of light is the same for any observer in any inertial frame, no matter how he or she is moving relative to the source of the light, and it is impossible to accelerate an object from below the speed of light up to the speed of light. All of these predictions have been tested and verified many times in experiment. It is the special theory that says that mass and energy can be interchanged in line with Einstein’s equation E0 = m0c2; this too has been confirmed by experiments.

6. General theory of relativity General theory of relativity: Theory of gravity developed by Albert Einstein in the early part of the 20th century, and presented to the Prussian Academy of Sciences in 1915. Because gravity is the dominant force in the universe at large (thanks to its very long range), the theory is also a theory of cosmology, and underpins all modern models of how the Universe got to be the way it is. Einstein’s special theory of relativity, published in 1905, deals with the dynamical relationships between objects moving at constant speeds in straight lines. It does not deal with accelerations, or with gravity, which is why it is called the “special” (meaning “restricted”) theory. Einstein always intended to generalize his theory to deal with accelerations and gravity, but it took him ten years (not all the time devoted exclusively to the general theory) to find a satisfactory mathematical description of the dynamics of the Universe and everything in it. Indeed, the whole point about Einsteins’s theory is that it gives us a physical picture of how gravity works; Isaac Newton discovered the inverse square law of gravity, but explicitly said that he offered no explanation of why gravity should follow an inverse square law. The general theory of relativity also says that gravity obeys an inverse square law (except in extremely strong gravitational fields), but it tells us why this should be so. That is why Einstein’s theory is better than Newton’s even thought it includes Newton’s theory within itself, and gives the same “answers” as Newton’s theory everywhere except where the gravitational field is very strong.

7. Practices, demonstrations Practices in special theory of relativity - Lorentz’s transformation, Lorentz’s matrix, relativistic factor, … Practices in general theory of relativity - description of the curved spacetime, metric tensor, Schwarzschild’s solution, … some demonstrations: http://demonstrations.wolfram.com/SchwarzschildSpaceTimeEmbeddingDiagram/ http://demonstrations.wolfram.com/GravitationVersusCurvedSpacetime/ http://demonstrations.wolfram.com/BendingOfLightByAStar/

8. General theory of relativity • Observed effects of general theory of relativity: • advance of perihelion • of Mercury • of binary pulsars • deflection of light • by Sun, observed first in 1919 by Arthur Eddington • gravitational lensing, from stars (low probability) and galaxies – often observed • gravitational time dilatation • gravitational redshift – direct observed by Pound & Rebka experiment • atomic clock shift by flight experiments • GPS … in fact permanent operating general relativity experiment • gravitational radiation • indirect observations in binary pulsars

9. General theory of relativity Advance of perihelion of Mercury The orbit of Mercury around the Sun does not trace out the same path every time, but shifts slightly from one orbit to the next. Each orbit is an ellipse, with the Sun at one focus of the ellipse. In each orbit, at the closest approach of Mercury to the Sun (perihelion), the ellipse shifts sideways by a tiny amount. This advance of the perihelion was predicted by Albert Einstein’s general theory of relativity but cannot be explained by Isaac Newton’s theory of gravity. The perihelion precession of Mercury is 5600 arc seconds per century. Newtonian mechanics, taking into account all the effects from the other planets, predicts a precession of 5557 seconds of arc per century. In the early 20th century, Albert Einstein’s General Theory of Relativity provided the explanation for the observed precession. The effect is very small: the Mercurian relativistic perihelion advance excess is just 42.98 arcseconds per century, similar, but much smaller, effects operate for other planets: 8.62 arcseconds per century for Venus, 3.84 for Earth, 1.35 for Mars (http://en.wikipedia.org/wiki/Mercury_(planet)) In 1859, the French mathematician and astronomer Urbain Le Verrier reported that the slow precession of Mercury’s orbit around the Sun could not be completely explained by Newtonian mechanics and perturbations by the known planets. He suggested, among possible explanations, that another planet (or perhaps instead a series of smaller 'corpuscules') might exist in an orbit even closer to the Sun than that of Mercury, to account for this perturbation. (Other explanations considered included a slight oblateness of the Sun.) The success of the search for Neptune based on its perturbations of the orbit of Uranus led astronomers to place faith in this possible explanation, and the hypothetical planet was even named Vulcan. However, no such planet was ever found. (http://en.wikipedia.org/wiki/Mercury_(planet))

10. General theory of relativity Deflection of light One of first key tests of the general theory of relativity which predicted that light from a distant star passing close by the Sun would be “bent” by a certain amount. The only way to observe this is during an eclipse, when the bright light of the Sun itself is blocked by the Moon, and stars can be seen on the sky around the eclipsed Sun. Albert Einstein published his paper predictihg this effect in 1916, abd British astronomer Arthur Eddington organiyed an expedition to observe the eclipse from Principe, off the west coast of Africa, while a second team observed it from Brazil. Photographs of the positions of the stars near the Sun on the sky were then compared with photographs of the same part of the sky taken 6 months earlier, when the Earth was on the other side of the Sun in its orbit and those stars were visible at night. The comparison showed that the stars photographed during the eclipse seemed to have been shifted sideways slighthly by the deflection of light – by exactly the amount that Einstein had predicted. In principle, this deflection occurs whenever light passes near a massive object, although usually the effect is too small to be measurable. It is caused by the curvature of spacetime associated with the mass. A more extreme version of the same effect causes the gravitational lens phenomenon, and in the ultimate extreme light is trapped completely within a black hole.

11. General theory of relativity Gravitational time dilatation, gravitational red shift Slowing down of clocks caused by a gravitational field, as predicted by the general theory of relativity. ω2 ω1 Φ2 Φ1 (linear approximation) With this effect must be for example calculated also by GPS navigation system ω1= 10,23000000000MHzω2= 10,22999999543 MHz (also included effect of special theory of relativity – transversal Doppler shift).

12. General theory of relativity • Binary pulsars • A binary pulsar exists when two neutron stars, one of which is a pulsar, are in orbit around one another, forming a binary star system. The term is also used to refer to a pulsar in orbit about any other star: for example, a white dwarf. More than twenty binary pulsars are now known, but astronomers reserve the term “the binary pulsar” for the first one to be discovered, which is also known by its catalogue number, as PSR 1913+16. This pulsar has provided the most accurate test yet of Albert Einstein’s general theory of relativity, and is the most accurate clock yet discovered. • The binary pulsar was discowered in 1974 by Russell Hulse and Joseph Taylor, of the University of Massachusetts, working in the Arecibo radio telescope in Puerto Rico. • Measured effects: • advance of the perihelion • time dilatation (STR effect) • losing energy as a result of gravitational radiation • Fro this results Hulse and Taylor won the 1993 Nobel Prize in Physics. • For other information see http://en.wikipedia.org/wiki/PSR_B1913%2B16

13. Equivalence principle Equivalence principle That the effects of acceleration are indistinquishable from the effects of a uniform gravitational field. This equivalence results from the equivalence between gravitational mass and inertial mass. It led Albert Einstein to the development of general theory of relativity, when he realized that a person falling from a roof would not feel the effects of gravity – the acceleration of their fall would exactly cancel out the feeling of weight. Acceleration is equivalent to a uniform gravitational field In modern language, the equivalence is best described in terms of a spaceship being accelerated through space by constant firing of its rocket motors. When the motors are not firing, everything inside the spaceship floats about in free fall, just as weightless as the person falling from a roof. In principle, the acceleration of the rocket could be adjusted so that everything inside felt a force exactly as strong as the force of gravity on Earth (or any other strength you chose), pushing things to the back of the vehicle as it moved forward through space. Any scientific experiments carried out in this accelerating frame of reference would give exactly the same results as if the spaceship were standing on its launch pad on Earth, and not accelerating at all.

14. Schwarzschild metric Eukleidian metric in kartesian and polar coordinates Schwarzschild metric (space out of spherical symetrical spread matter) Minkowski metric in polar coordinates (space without matter or in special theory of relativity) Where is the Schwarzschild radius. You can see that the Minkowski metric is limit of the Scwarzschild metric for for r>> rg.

15. Friedman metric Friedman metric (space with matter regularry spreded – with constant density) You can see that the Minkowski metric is limit of the Scwarzschild metric for for r>> rg.

16. The final test … will take place the last week of semester, on Tuesday 17.5.2001 at 16:15 after the last lecture, during the time for practise in the room T2:C2-81. Later you can only arrange the alternative date and time of the final test with me individually For other information see http://www.aldebaran.cz/~zacek/education/astrophysics_final_test.html 12 questions, each have 4 possibilities, maximum gain is 48 points,