Physics Question #2867

George Chronis, a 34 year old male from Toronto asks on August 4, 2005,

I have a few questions regarding Space-Time that have always confused me. I would very much appreciate if you could provide some insight: 1.Expanding Universe It has been discovered that the universe is expanding at an accelerating rate. This conclusion was based by studying the light of a special class of supernovae in different galaxies. Is it possble that we could come to a different conclusion if we were in another region of space-time? For instance, would not someone situated in/near a black hole see the universe evolving at a fantastic rate? Therefore, is it not possible that the observation that the universe is expanding at an accelerating rate is biased by our local and our motion in/near an intense gravitational field? 2.Cosmos Cartography The universe has been observed as matter existing in "thin filaments" with "large pockets" of empty space. Is this an acurate picture of the universe "now" (with respect to the observer)? When creating such a cartography of the cosmos, was the correlation between distance and time taken into account?; namely, the more distant an object the further back in time the object is observed. For instance, suppose someone on earth was to create a map of the surface of the earth. Furthermore, suppose he travels 1million years back in time per 1000km of radius travelled (from an arbitrary starting point). By the time he has traveled a distance equal to the earth's radius, he would have travelled over six million years into the past. If he were to create a map of the earth based on what he observed at every circle, his map would be one of geography and history (at the "equator" the solar system had not formed, at the midpoint life would just be starting on earth). Therefore, would this map represent the earth the way it is "now"? Similarly, is the "map" of the universe as based on "old data" (light seen from the night sky) the "true picture" of the universe "now". 3.Galaxy Dynamics It has been observed that the outer portions of a galaxy spin faster than inner core of a galaxy. Several theories have been proposed: a ring of dark matter circling the galaxy, modifying the 1/R2 term in Newton's gravitation equation. Could the observed slower spin at the galaxy core be explained by the General Theory of Relativity? The intense gravitational field at the galaxy core would make events at the core appear, to observers further away, slower. 4.The Big Bang A few moments after the big bang, is some times described in spatial terms. For example, at 10^-47 seconds after the big bang, the size of the universe was smaller than an atom. If at 10^-47 seconds after the big bang all of space and time existed at that instant including all matter and energy, can one asign a size at that point in time drawing from the state of the universe as it is today (i.e. what exactly does "smaller than an atom" mean when the universe is just 10^-47 seconds old?) ? Furthermore, can one talk of time near the big bang? By asigning a time, do you not in essence establish absolute time (i.e. time zero becomes an absolute time reference) in contradiction to Theory of Relativity.

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The answer

Doug Hube answered on August 17, 2005

1.Expanding Universe

It is true that the strength of a gravitational field affects measurements of space and time. However, except at discrete points occupied by black holes, and perhaps other extreme objects if one is very close to a surface, our location is in a 'weak field situation' as is most of the Universe. We are not in a region where the suggested biases could play a role.

2.Cosmos Cartography

The 'filamentary' and 'bubbly' structure of normal matter has been detected within the relatively 'small' distance of a few billion light years. The scale of the filaments and bubbles/voids is such that, the relative motions of the galaxies which define those structures are too small to have significantly altered the structure during the equivalent few billion years.

3.Galaxy Dynamics

Yes, most spiral galaxies that have been studied appropriately do not follow a Keplerian velocity distribution in their outer regions. That is, rather than the rotational velocity decreasing with increasing distance from the galaxy's centre (as the orbital speeds of planets decrease with increasing distance from the Sun) the velocities either remain roughly constant or, in some cases, increase with distance from the centre of the galaxy. The usual explaination is that there is a large halo of dark matter.

There do seem to be massive black holes at the centres of most galaxies, but they are very much smaller than what we call the 'core'. Relativistic effects will be signifivant very close to those nuclear black holes but very insignificant throughout the much greater volume of a typical core. The cores of galaxies constitute 'weak field' regions.

4.The Big Bang

Yes, the Big Bang event does define a zero point in time ... cosmic time.

Special Relativity says that a clock in one inertial reference frame runs 'slowly' as measured with respect to a clock in another inertial reference frame. A full discussion leads one into a consideration of the meaning of 'simulaneity', etc.

We are not observing the 'cosmic clock' from another reference frame, i.e. from outside the Universe. The 'clocks' and the measurement of their rates are fundamentally different in those two situations. We are integral components of the cosmic structure that is evolving in step with the 'ticks' on the cosmic clock which began to 'tick' 13**9 years ago, just as my 'ticking heart clock' began at a well-defined time as measured by clocks in my local not-relativistically-affected corner of the Universe.

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