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    B How can dark energy comprise 74% of the Universe?

    But they are related contrary to your statement. Light has to cross the Hubble sphere to reach us. See section 3.3...
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    B How can dark energy comprise 74% of the Universe?

    Yes. But how far we can see is related to... I agree you can argue they have the same underlying cause. I have learnt my lesson. Regards Andrew
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    B How can dark energy comprise 74% of the Universe?

    Ok I give up. @PeterDonis why say this You now say this I explicitly did not equate them at the same time. For the last time I said the expanding Hubble sphere will mean we can see further (than the currently observable Universe) in the future. Regards. Andrew
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    B How can dark energy comprise 74% of the Universe?

    The current observable Universe is how far we can see now. That is not how far we will be able to see in the future. Which is what I said. Please point out where I equated the two. As the Hubble sphere is expanding we will be able to see further in the future. I also know we can see things...
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    B How can dark energy comprise 74% of the Universe?

    I know that. I never said if was. Regards Andrew
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    B How can dark energy comprise 74% of the Universe?

    As I understand it the Hubble sphere is expanding so in the future we will be able to see further than current size of the observable Universe. Regards Andrew
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    I Transparency of a gas compared to a plasma

    The spectra of the light emitted at the photosphere of the sun , as an example, matches that received at the top of our atmosphere very accurately with small additions from light from the chromosphere and corona. Regards Andrew
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    I Transparency of a gas compared to a plasma

    If there are strong magnetic fields to accelerate free electrons in a plasma then they will emit cyclotron radiation. Also significant quantities of hot gas (~10% by mass) were missed in galaxies until they were observed in x-rays. Regards Andrew
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    A GR vs. alternative theories of gravity

    As per the summary I would like to find a review paper, monograph or book. Google is not find any recent reviews. Post 2018 would be ideal. Thanks for any pointers. Regards Andrew
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    I Some questions about absorption lines

    Q1 the spectrum is cut up and stacked. Take the right hand edge of the strip below and attach it to the left hand edge to the one above and so on. Q2 yes the is normally some signal in the dark bands. Q3 Yes it remitted but in all directions and not all back to it's original trajectory. Q4...
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    I Astronomy in a Simple Solar System

    I am not disputing your technical definition but given the baricenter of the Earth Moon system is inside the Earth a Galileo observing from Mars might conclude the moon orbits the earth at least informally. Regards Andrew
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    I Astronomy in a Simple Solar System

    I suspect we are conflating the technical and everyday definitions of orbit. When I observe the Galilean moons of Jupiter I am with Galileo in that they appear to orbit it. Regards Andrew
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    I Astronomy in a Simple Solar System

    Ptolemy epicycles always struck me as an precursor to Fourier decomposition. Regards Andrew.
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    I Astronomy in a Simple Solar System

    @Ibix Ptolemy would be proud of you! Regards Andrew
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    I Astronomy in a Simple Solar System

    You have repeated this several times but I don't understand it. In an Earth centerd model the all solar system bodies orbit the earth in the sense the perform circuits. Yes the are very complex but so is the motion of the moon around the Sun it is not a simple ellipse. Don't get me wrong I am...
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    I Astronomy in a Simple Solar System

    You would still need a reference frame. If you centred it on the earth the Sun would orbit it and vice versa. If you used the fixed stars then yes your right the earth would orbit Sun but the theory is then then stars are fixed. You can't escape motion is relative. Regards Andrew
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    I Astronomy in a Simple Solar System

    Yes I agree and understand all that, and it is the "obvious choice" but that's still an example of a particular choice motivated by simplicity of the equations of motion i.e. the point I am making. Theory leads to it being the "obvious choice" not raw observation. Kepler found he could...
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    I Astronomy in a Simple Solar System

    I agree but I was trying to reinforce the point I made in post #27. You either need to appeal to "simplicity " or a fixed reference frame ( stars, QSO etc.) to establish a barycentric system. This is the lesson relativity is it not? Regards Andrew
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    I Astronomy in a Simple Solar System

    But can't you transform the equations of motion from a barycentric system to an earth centred one? Yes they would be more complex but would predict the same outcome. Regards Andrew
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    I Astronomy in a Simple Solar System

    In reality don't you have to make some assumption about a frame of reference for example the fixed stars otherwise you have to appeal to simplicity of explanation. If you don't, you don't have any reason to choose a particular frame. Regards Andrew
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    B The composition of interstellar dust

    A quick Google turns up lots of links which might answer your questions. Regards Andrew
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    I Alfvén wave observation

    I don't know the answer to your question but just to observe the zeeman splitting requires and R of 30000 or above. Given the atmosphere I think it would be very difficult with typical amateur equipment. Regards Andrew
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    B NASA discovers water on the moon!

    Good job Neil Armstrong and co took there boots!
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    Biological Agent Phosphine Found on Venus

    A re analysis of the alma data finds no evidence for phosphine Re analysis Regards Andrew
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    Biological Agent Phosphine Found on Venus

    If I recall correctly they did consider sulphur dioxide as a possibility but dismissed it. You also have to consider the conditions which can broaden the lines, various relative velocities (wind speed, Earth Venus relative velocity) which can shift the lines so it is not as absolute as you might...
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    Biological Agent Phosphine Found on Venus

    Yes wavelength is accurately measured but assignments us a different problem. There are many lines in high resolution spectra and a single line is not reliable proof . Hence the need for confirmation. This is standard practice and does not challenge all of spectroscopy. Regards Andrew
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    I What happens to a small star after burning all of its fuel?

    The life time is so long I don't think we have any observation of the end of life of red dwarf stars yet. Brown dwarfs fizzel out after converting deuterium to helium. Regards Andrew
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    Insights How to Recognize Split Electric Fields - Comments

    What about the Aharonov-Bohm effect due to the EM potentials Regards Andrew
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    Stargazing The curse of Elon Musk

    Take multiple images and stack using median combine and that should remove them. Regards Andrew
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    I How did the initial dust particles form?

    Both, on a large scale you can treat the molecular cloud classically. If you want to look at the details of particle interacting then QM might be more accurate although even here classical approximations may be good enough. In colapse to stars gravity is the driver with em getting rid of...
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