AboutJames Gort Expertise Questions on observational astronomy, optics, and astrophysics. Specializing in the evolution of stars, variable stars, supernovae, neuton stars/pulsars, black holes, quasars, and cosmology.
Experience I was a professional astronomer (University of Texas, McDonald Observatory), lecturer at the Adler Planetarium, professor of astrophysics, and amateur astronomer for 42 years. I have made numerous telescopes, and I am currently building one of the largest private observatories in Canada.
Publications StarDate, University of Texas, numerous Journal Publications
One of my biggest question is about the size of the observable universe.
In one of your answers to Steve (nighttrain@nyc.rr.com) you said that "In reality, we can see only about 10 billion light years away, where galaxies are receding at more than 50% the speed of light. As we push the distance further, the recession velocity gets higher, so we could never see the "beginning". It is thought that about 12 BILLION LIGHT YEARS is the limit at which we can observe - light simply "disappears" as it gets red-shifted to the extreme.
Based on your above explanation, I want you to answer my simple question:
Is it correct that beyond 12 BILLION LIGHT YEARS in all directions, an object's recessional velocity EXCEEDS the velocity of light??
Thank You
Indro
Jakarta, Indonesia
ANSWER: Hello Indro,
As the red-shift gets greater, it appears that Hubble's velocity / distance "law" is not linear. Due to relativistic effects, the velocity only approaches "c" as the distance approaches about 13 billion light years. But you can't really talk about distances the way we're used to - because space itself was compressed. The "big bang" is an expansion of space-time itself, so gravitational red-shifts play an increasingly important role in the early universe, and rulers would be "shrunk" (as seen by us) in that highly curved region of long ago. So to answer your question, Einstein predicted that "c" was a limiting velocity, and observations of distant galaxies only seem to confirm that. Unless relativity is modified, there is no way that galaxies can recede at a speed greater than "c".
But all this is very speculative. If you read my past answers, you might have seen my recommendation to read Halton Arp's book "Seeing Red", in which he makes some very good arguments for other causes of the red-shift (besides recessional velocity). If he turns out to be correct, Hubble's "law" would be quite wrong.
Prof. James Gort
---------- FOLLOW-UP ----------
QUESTION: Thanks for your answer.
But in a 13-billion-years-old universe, is it possible that an object's recessional velocity equals the speed of light at a distance of less than 13 billion light years (12 or 11 or 10 billion light years for example) ?
Thanks again
Answer Hello Indro,
Although I'll never say "never", from what we know of physics today (mainly from Einstein), much of which has been experimentally proven, no mass can have a relative velocity from an observer at greater than the speed of light. To do so would require an infinite amount of energy. That doesn't mean it's impossible. We know very little about quantum gravity. And one of the most successful theories in physics, Quantum Electrodynamics (QED), predicts that individual photons can (and do) exceed the "average speed of light". I refer you to a little book (but extremely good) by Nobel Laureate Richard Feynman entitled "QED - The Strange Story of Light and Matter". That will open your eyes to many possibilities!
