# OPTION E - ASTROPHYSICS E4 Cosmology Olber’s Paradox.

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OPTION E - ASTROPHYSICS E4 Cosmology Olber’s Paradox

E.4.1 Describe Newton’s model of the universe. Newton assumed an -infinite (in space and time), -uniform and -static universe (otherwise it would collapse under its own gravitational force). The universe was unchanging and it would contain an infinite number of stars spreading out to infinity.

Why isn't the night sky as uniformly bright as the surface of the Sun? If the Universe has infinitely many stars, wherever you looked in the night sky you would see a star. Then the night sky should be bright. Why is the night sky dark? or E.4.2 Explain Olbers’ paradox.

Very distant stars contribute with very little light to an observer on Earth but there are many of them. So if there is an infinite number of stars, each one emitting a certain amount of light, the total energy received must be infinite, making the night sky infinitely bright, which is not. E.4.2 Explain Olbers’ paradox. If the Universe is eternal and infinite and if it has an infinite number of stars, then the night sky should be bright.

If we consider the Universe finite and expanding, the radiation received will be small and finite mainly for 2 reasons: There is a finite number of stars and each has a finite lifetime (they don’t radiate forever) and Because of the finite age of the Universe, stars that are far away have not yet had time for their light to reach us. Also, The Universe is expanding, so distant stars are red- shifted into obscurity (contain less energy). E.4.2 Explain Olbers’ paradox.

E.4.2 Explain Olbers’ paradox – quantitative explanation

OPTION E - ASTROPHYSICS E4 Cosmology The Big Bang Model

In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Also, because of the inverse relationship between frequency and wavelength, we can describe the Doppler shift in terms of wavelength. Radiation is redshifted when its wavelength increases, and is blueshifted when its wavelength decreases. E.4.3 Suggest that the red-shift of light from galaxies indicates that the universe is expanding.

In astronomy, the Doppler effect was originally studied in the visible part of the electromagnetic spectrum. Today, the Doppler shift, as it is also known, applies to electromagnetic waves in all portions of the spectrum. Astronomers use Doppler shifts to calculate precisely how fast stars and other astronomical objects move toward or away from Earth. E.4.3 Suggest that the red-shift of light from galaxies indicates that the universe is expanding.

Why is Doppler effect so important? In 1920’s Edwin Hubble and Milton Humanson realised that the spectra of distant galaxies showed a redshift, which means that they are moving away from Earth. So, if galaxies are moving away from each other then it they may have been much closer together in the past. Matter was concentrated in one point and some “explosion” may have thrown the matter apart. E.4.3 Suggest that the red-shift of light from galaxies indicates that the universe is expanding.

Big Bang The Big Bang Model is a broadly accepted theory for the origin and evolution of our universe. It postulates that 12 to 14 billion years ago, the portion of the universe we can see today was only a few millimetres across. It has since expanded from this hot dense state into the vast and much cooler cosmos we currently inhabit. We can see remnants of this hot dense matter as the now very cold cosmic microwave background radiation which still pervades the universe and is visible to microwave detectors as a uniform glow across the entire sky.

Big Bang The singular point at which space, time, matter and energy were created. The Universe has been expanding ever since. Main evidence: Expansion of the Universe – the Universe is expanding (redshift)  it was once smaller  it must have started expanding sometime  “explosion” Background radiation  evidence of an hot Universe that cooled as it expanded Helium abundance  He produced by stars is little  there is no other explanation for the abundance of He in the Universe than the Big Bang model.

In 1960 two physicists, Dicke and Peebles, realising that there was more He than it could be produced by stars, proposed that in the beginning of the Universe it was at a sufficiently high temperature to produce He by fusion. In this process a great amount of highly energetic radiation was produced. However, as the Universe expanded and cooled, the energy of that radiation decreased as well (wavelength increased). It was predicted that the actual photons would have an maximum λ corresponding to a black body spectrum of 3K. So, we would be looking for microwave radiation. E.4.5 Describe the discovery of cosmic microwave background (CMB) radiation by Penzias and Wilson.

Shortly after this prediction, Penzias and Wilson were working with a microwave aerial and found that no matter in what direction they pointed the aerial it picked up a steady, continuous background radiation. E.4.5 Describe the discovery of cosmic microwave background (CMB) radiation by Penzias and Wilson.

In every direction, there is a very low energy and very uniform radiation that we see filling the Universe. This is called the 3 Degree Kelvin Background Radiation, or the Cosmic Background Radiation, or the Microwave Background. These names come about because this radiation is essentially a black body with temperature slightly less than 3 degrees Kelvin (about 2.76 K), which peaks in the microwave portion of the spectrum. E.4.5 Describe the discovery of cosmic microwave background (CMB) radiation by Penzias and Wilson.

Why is the background radiation an evidence for the Big Bang? The cosmic background radiation (sometimes called the CBR), is the afterglow of the big bang, cooled to a faint whisper in the microwave spectrum by the expansion of the Universe for 15 billion years (which causes the radiation originally produced in the Big Bang to redshift to longer wavelengths). E.4.6 Explain how cosmic radiation in the microwave region is consistent with the Big Bang model.

If galaxies are moving away from us in all directions then the radiation reaching us from them will be red-shifted owing to Doppler effect. This explains why the sky appears dark at night: the light from receding stars has been shifted to the infra-red region of the electromagnetic spectrum and so is no longer visible to us. The Big Bang model therefore resolves Olber’s Paradox. E.4.7 Suggest how the Big Bang model provides a resolution to Olbers’ paradox.

OPTION E - ASTROPHYSICS E4 Cosmology The development of the universe

Universe ClosedOpen The expansion of the Universe will eventually stop and the universe will start to collapse (Big Crunch). The Universe will expand forever at a slowing rate. Flat The Universe will expand forever at a slowing rate that approaches zero. E.4.8 Distinguish between the terms open, flat and closed when used to describe the development of the universe.

E.4.9 Define the term critical density (ρ C ) by reference to a flat model of the development of the universe. So, how do we measure the density of the Universe?

E.4.10 Discuss how the density of the universe determines the development of the universe. Universe ClosedOpenFlat

If we take in account all the matter (stars) that we can see then the total mass would not be enough to keep the galaxies orbiting about a cluster centre. So, there must be some matter that can not be seen – dark matter. This dark matter cannot be seen because it is too cold to irradiate. According to the present theories dark matter consists in MACHO’s and WIMPS. E.4.11 Discuss problems associated with determining the density of the universe.

MACHO’s WIMP’s Massive compact halo objects – brown and black dwarfs or similar cold objects and even black holes. Non-barionic weakly interacting massive particles (neutrinos among other particles predicted by physics of elementary particles). It seems that there is also what is called and dark energy… The increase of the potential energy reduces the kinetic energy of the parts of the universe, causing the expansion to slow down. However, if there was some sort of negative energy it would have the positive effect. This is called dark energy E.4.11 Discuss problems associated with determining the density of the universe.

E.4.13 Discuss an example of the international nature of recent astrophysics research. Outline any astrophysics project that is funded by more than one country. E.4.14 Evaluate arguments related to investing significant resources into researching the nature of the universe. Understand the issues involved in deciding priorities for scientific research. Express your own opinions coherently.

Cosmic Journeys - How Large is the Universe? https://www.youtube.com/watch?v=yaX4iGw-b_Y Cosmic Journeys - How Large is the Universe? https://www.youtube.com/watch?v=yaX4iGw-b_Y