MadSci Network: Astronomy

Re: do we know what the universe looks like in the present?

Date: Wed Sep 1 17:53:27 1999
Posted By: Denise Kaisler, Grad student, Astronomy, UCLA, Division of Astronomy
Area of science: Astronomy
ID: 935698259.As

Hi Michael,

It seems to me as if you've made a close association between two ideas
which are actually separate. Let me explain the way I see it.

The first concept you mentioned is that of "lookback time": the further
away an astronomical object is, the longer it takes light from that object
to reach us. Despite what many people think, this phenomenon crops up
whenever we look at something. We just don't notice it every day because
light crosses the distance between fishes, Ferraris and other earthbound 
objects almost instantaneously. Only when light has to cross enormous
regions of space does it become a concern.

This means that when we look at distanat galaxies, we see them not as
they appear today, but as they were when the photons arriving now left
their source.

In a sense, you are right about being uncertain about the present state
of a star or galaxy. If the light from the star Vega took 26 light years
to reach us, how do we know that Vega is still there? Twenty-six years
is a long time. What if Vega went supernova in that time, or aliens fed
it into a big star-compacting machine? 

The answer to this is an easy one. Astronomers assume, based on the
million or billion-year lifetimes of stars and galaxies, that nothing
much has gone on since the light we're now seeing left its source.

But this assumption breaks down for very distant objects such as
quasars, which are billions of light years away. A lot more could have
happened in a billion years, so it's not safe to assume that quasars are
still as we see them. Instead, we assume that the quasars have evolved
into galaxies resembling our own. But we're more uncertain about this
than we are about nearby stars, hence the greater uncertainty that you 

Yet this is *not* linked to Heisenberg's uncertainty principle, which
describes the probabilistic behaviour of very small particles like
protons and electrons. This tenet of quantum mechanics does have to
do with light and how we sense things, it is on an entirely different
scale from lookback time.

Now, that that's been cleared up, the final part of your question awaits.
The answer is no, it's impossible to really know what those
very distant quasars look like right at this minute. All we can
do is assume that they have evolved into galaxies similar to those
in the Local Supercluster. My answer is based on the fact that light
travels at a constant speed for all comoving observers. Nobody has
yet thought of a way to get around this. 

If you want to win a Nobel Prize, see if you can come up with a way
to see the far reaches of the universe as they appear right now.
I, for one, would be really interested.

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