"Light from the Most Ancient, Distant Galaxies will Never Reach Earth"

 

          

New research finds that the ideal time to study the cosmos was more than 13 billion years ago, just about 500 million years after the Big Bang - the era (shown in this artist's conception above) when the first stars and galaxies began to form. Since information about the early universe is lost when the first galaxies are made, the best time to view cosmic perturbations is right when stars began to form. Modern observers can still access this nascent era from a distance by using surveys designed to detect 21-cm radio emission from hydrogen gas at those early times.

The galaxies and larger-scale structures that have evolved over its 13.7-billion-year history began as small perturbations of matter that grew over time, like ripples in a pond, as the universe expanded. By observing the large-scale cosmic wrinkles now, we can learn about the initial conditions of the universe. But is now really the best time to look, or would we get better information billions of years into the future - or the past?

New calculations by Harvard theorist Avi Loeb show that the ideal time to study the cosmos was more than 13 billion years ago, just about 500 million years after the Big Bang. The farther into the future you go from that time, the more information you lose about the early universe."I'm glad to be a cosmologist at a cosmic time when we can still recover some of the clues about how the universe started," Loeb said.

Two competing processes define the best time to observe the cosmos. In the young universe the cosmic horizon is closer to you, so you see less. As the universe ages, you can see more of it because there's been time for light from more distant regions to travel to you. However, in the older and more evolved universe, matter has collapsed to make gravitationally bound objects. This "muddies the waters" of the cosmic pond, because you lose memory of initial conditions on small scales.

The two effects counter each other - the first grows better as the second grows worse.Loeb asked the question: When were viewing conditions optimal? He found that the best time to study cosmic perturbations was only 500 million years after the Big Bang.

This is also the era when the first stars and galaxies began to form. The timing is not coincidental. Since information about the early universe is lost when the first galaxies are made, the best time to view cosmic perturbations is right when stars began to form.

But it's not too late. Modern observers can still access this nascent era from a distance by using surveys designed to detect 21-cm radio emission from hydrogen gas at those early times. These radio waves take more than 13 billion years to reach us, so we can still see how the universe looked early on.

"21-centimeter surveys are our best hope," said Loeb. "By observing hydrogen at large distances, we can map how matter was distributed at the early times of interest."

The accelerating universe makes the picture bleak for future cosmologists. Because the expansion of the cosmos is accelerating, galaxies are being pushed beyond our horizon. Light that leaves those distant galaxies will never reach Earth in the far future. 

In addition, the scale of gravitationally unbound structures is growing larger and larger. Eventually they, too, will stretch beyond our horizon. Some time between 10 and 100 times the universe's current age, cosmologists will no longer be able to observe them.

"If we want to learn about the very early universe, we'd better look now before it is too late!" Loeb said.

More information: This research was published in the Journal of Cosmology and Astroparticle Physics (JCAP) and is available online.

The Daily Galaxy via Harvard-Smithsonian Center for Astrophysics   

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Comments

[Narendra Nath]

Purely on conceptual basis, i also see the great importance of the first half a billion years of the universe. Two mss ' Mysteries of the Universe' and ' Inconstancy of the Physical Constants and Strengths of the force/fields' pen down a few years back, indicate the very things this article indicates through a proper scientific study. In fact, Physics of those early times can not be described by today's Physics postulated in the past 50 decades of activity!I was then told that that period can not be studied at all as e.m. signals were not generated at that time. The present study however mentions the study of 21-cm radiations at the time the first stars were being formed from elemental hydrogen then.. Hope such studies are now undertaken in right earnest.

[small brain]

I really struggle to understand our observations of the early universe. If the light from the early universe has taken 13 billion odd years to reach us - was the universe expanding at a more rapid rate than the speed of light for a good percentage of that time? Sorry to be stupid, I just have a real block with this. If anyone can explain it in simple terms I'd appreciate it.

[Bob Danforth]

The thing is that objects at great distances right after the big bang have sent their light in our direction when the universe was half or less the size it was by the time the light was received here, but at any given time some of the light is only halfway, and thus not needing to exceed the speed of light, even though the object itself is receding at beyond the speed of light.

Therefore it would seem to me that as objects continued to accelerate the redshift would continue down the spectrum so that x-rays would be seen as microwaves to long radio waves long after the universe was many times its current size and closer galaxies would take many times longer before they were red shifted too the point that x-rays were long radio waves.

Since the Deep Field Galaxies are still well in the visible range, a microwave level image in that detail should reach to well before the big bang, or rather to the earliest time that light could cross the universe at all.

Because of this the observer would always appear to be at that "lucky confluence" when the outer extant of the universe was just at the maximum range of visibility. I do not know it this effect has already been calculated for or exactly the effect it would have but it might make the universe appear to be accelerating.

[Ins0mniac]

small brain,

I'm not an expert but I remember hearing that space-time itself does expand faster than the speed of light. The "speed of light" limit only applies things WITHIN space-time not space-time itself.

[James Ph. Kotsybar]

“DARK BACKWARD AND ABYSM”
-- James Ph. Kotsybar

We can see fourteen billion light years out.
For those still here a billion years from now,
more light will have traveled to them, no doubt,
the billion light years that space will allow.
Distant descendants may not see much more,
however, than what we can now observe.
Despite larger radius to explore,
their view won’t be a sight they can conserve,
because space itself goes faster than light,
as it expands relatively through time.
This perspective's loss is ever the plight
throughout our universe's known lifetime.
We daily lose ability to see
the things furthest back in our history.

[perrenod]

@small brain - The galaxies that we can observe emmitting their light from 13 billion years ago are moving away from us at less than the speed of light, but at a considerable fraction of the speed of light. Thus their light, originally emitted in the visible portion of the spectrum is highly redshifted into the infrared. And the 21-cm hydrogen line radio emission is redshifted to considerably longer wavelengths (lower frequencies). Since as Ins0mniac correctly recalls, general relativity allows space itself to be stretched at faster than light speeds (for objects sufficiently separated) there are other galaxies that we can never detect.They are beyond our "light horizon". But the ones under consiideration in this article are within that light horizon and receding from us at a substantial fraction, but less than, the speed of light, c.

[Mark D]

Are there other galaxies we can never detect? Even if they are moving away from us faster than the speed of light, why can their light emissions never reach us at some point.

Strange things happen at that speed and even stranger things happen at greater speeds if they exist. I think this is still conjecture. Equally, I don't think the proof is in that the universe is 13.7 billion years old or that it is finite.