# cosmolgy

1. ### I Hubble relation to Scale Factor

Imagine a Universe where the Hubble parameter is truly a constant, in both space and time. How much smaller would such a Universe be 14 billion years ago compared to today? Using the Hubble parameter in terms of scale factor: $H(t) = \frac{\dot{a}}{a}$ leads to the differential equation...
2. ### I Could fundamental laws change in Dirac's Large Numbers Hypothesis?

Paul Dirac proposed a hypothesis called "Large Numbers Hypothesis" (https://en.wikipedia.org/wiki/Dirac_large_numbers_hypothesis), where he basically stated that, if he was correct, laws of physics would change with time. But what about fundamental laws and constants? (Not only 'effective'...
3. ### Did Paul Dirac believe in multiple universes?

Prominent physicist Paul Dirac proposed a hypothesis that indicated that constants and laws of physics would evolve with time into different constants and laws of nature (https://en.wikipedia.org/wiki/Dirac_large_numbers_hypothesis) This hypothesis was used by Robert Dicke...
4. ### A Does Bohmian Mechanics yield the existence of a multiverse?

David Deutsch, a theoretical physicist, talks about David Bohm in his book "the Fabric of Reality": "[w]orking out what Bohm’s invisible wave will do requires the same computations as working out what trillions of shadow photons will do. Some parts of the wave describe us, the observers...
5. ### A Would Wheeler's oscillatory universe model create different laws?

According to physicist John Wheeler, the universe is part of a sequence of cycles of Big Bangs and Big Crunches, called cyclic or oscillatory universe model (https://en.m.wikipedia.org/wiki/Multiverse (Mentioned in "Classification schemes") According to this author, in each cycle, the laws of...
6. ### Does Lawrence Krauss believe in the anthropic principle?

Summary: Does Lawrence Krauss believe in the anthropic principle? I have read a lot of texts both indicating that physicist Lawrence Krauss is against the anthropic principle. But I have read several texts indicating that Lawrence Krauss believes that this principle is true and has developed...
7. ### I Did Wheeler consider Carter's anthropic multiverse?

Physicist John Wheeler wrote an article called "Genesis and Observership" where he said near the end: "We have reviewed the evidence out of the big bang tat the universe did come into being and the evidence that not only space but all the structures and laws of physics are mutable. In the...
8. ### I Does the Strong Anthropic Principle have any equivalent specific model?

The anthropic principle is a philosophical consideration that observations of the universe must be compatible with the conscious and sapient life that observes it. There are two main types of anthropic principle: Weak Anthropic Principle (WAP) and Strong Anthropic Principle (SAP) The Strong...
9. ### I Are all Cellular Automata models of universes?

Both Stephen Wolfram and nobel laureate Gerard 't Hooft think that the universe is a Cellular Automata. As far as I know, 't Hooft developped a series of frameworks to build different models of Cellular Automata and Wolfram also proposed a framework where network nodes could produce different...
10. ### A Is the Wheeler-DeWitt equation related to Wheeler's It from Bit?

In this article: https://arxiv.org/pdf/hep-th/0211048.pdf by physicist Andrei Linde, it mentions Wheeler-DeWitt equation and he relates it to Wheeler's Participatory Universe model. Also, I found other sources that seem to establish a connection between Wheeler-DeWitt equation and Wheeler's It...
11. ### Cosmological Redshift

1. Homework Statement Consider a point in the intergalactic medium at some cosmic time $t_{obs}$, the time of arrival of a photon of wavelength $λ_{obs}$ as seen by a hydrogen atom at that location. The source of this photon a comoving distance $r$ away emitted it at wavelength...
12. ### What's book would you recommend on Cosmology?

I have searched and came across: Introduction to Cosmology by Barbara Ryden https://www.amazon.com/dp/0805389121/?tag=pfamazon01-20 Modern Cosmology by Dodelson https://www.amazon.com/dp/0122191412/?tag=pfamazon01-20 What would you recommend? This is for a Master's course where basic...
13. ### Can twistor theory be used to describe everything?

I've been looking at another Penrose's theory/model called twistor theory (https://en.wikipedia.org/wiki/Twistor_theory) that seems very interesting. From what I've read it seems to me that literally everything can be represented by twistors (or, rather, variations of twist theory). The only...
14. ### I What are the latest thoughts about the timescape cosmology

I came across a theory called "timescape" cosmology. If I understand the theory correctly it assumes that the universe is not homogenenous due to the large voids and gravitationally bounded system. Also time would be different for the observers on these two different locations. 1- Is CMBR and...
15. ### I Saddle and flat universe edges?

Hi, I have become very interested in cosmology recently and I had a question about the possible shapes of the Universe. I understand the critical density plays a role in the shape. This is what I know (or think I know). At the critical density of energy and matter, the universe is flat. If it...
16. ### I Harnessing energy from black holes?

Whether it be through Hawking radiation, miniature black holes, or even white holes, is it possible that one day energy could be harnessed from black holes and used on earth?
17. ### Gravitationally bound and Hubble time

1. Homework Statement Estimate how long a galaxy in the Coma cluster would take to travel from one side of the cluster to the other. Assume that the galaxy moves with a constant speed equal to the cluster’s radial velocity dispersion. How does this compare with the Hubble time, t H ? What can...
18. ### I How can there be fluctuations in the inflaton field value?

http://www.counterbalance.org/cq-turok/etern-body.html https://ned.ipac.caltech.edu/level5/Kolb/Kolb3_1.html) http://www.astronomy.ohio-state.edu/~dhw/A873/notes7.pdf) According to QFT, if you make repeated measurements of some property of the field then you will in general measure a...
19. ### Physics Starting my PhD in cosmology -- question about my career prospects

Hello all, Next year, January I will start my PhD in Cosmology. Salary is fair and I don't have to pay the tuition fee. I always loved the idea of doing a thesis about Cosmology. My master's degree was about computational physics where I had to learn how to program in C/C++, Fortran, Matlab...
20. ### I would like advice on which sub-areas I should follow for two Masters

Hi folks, My name is José Ricardo, I study at the Federal University of Rio de Janeiro (1st period). As the majority of the academics, always there is one who wants to follow a career. I'm doing major in Physics. I want to teach? Yes, I do. But I want to follow the research as well. I have...
21. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 1: Introduction

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
22. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 2: Kepler's Law

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
23. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 3: The Solar System

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
24. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 4: The Solar System (Contd.)

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
25. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 5: Binary Systems

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
26. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 6: Binary Systems (Contd.)

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
27. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 7: Tidal Forces and the Earth Moon System

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
28. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 8: Fluid Mechanics

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
29. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 9: Hydrostatics and the Solar Wind

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046
30. ### Astrophysics & Cosmology by Prof. S. Bharadwaj (NPTEL):- Lecture 10: Radiative Transfer

Copyright reserved to Prof. Bharadwaj and NPTEL, Govt. of India. Lectures: http://www.nptel.ac.in/courses/115105046/ Syllabus: http://www.nptel.ac.in/syllabus/syllabus.php?subjectId=115105046