You are here:

Astronomy/The "look-back time" concept


Hello, I am a chemistry undergraduate student.
I have recently encountered the concept of
"Look-back time" in an elementary astronomy course. I am quite confused about one thing. From my understanding, the greater the distance we look into the space the further we look back in time. If something is at a distance,say 1 billion light year from us, we observe that thing in 1 billion years ago. Then if that thing move from
1 billion light year to 10 billion light year distance from us, assume that during the travel, the thing keep evolving. What will we observe? Will we see that thing evolve backward to 10 billion years ago?
I hope I get the question clear. Thank you.

Expansion vs. time for universe
Expansion vs. time for  

What you are referring to is basically, cosmic expansion - the expansion of the universe. It is detected by what we call the "red shift" and in general this will apply to galaxy clusters, not individual galaxies. It will also apply to objects known as quasars or quasi-stellar objects.

The relationship of distance (or displacement) is by way of the recessional velocity and the Hubble constant, H. Then, the distance d = V/ H.

Given the Hubble constant is currently estimated at H  = 100 km/ sec/ Mpc

where Mpc denotes Mega-parsec (1 parsec = 3.26 LY)

This implies we would never see an object currently at 1b LY "evolve" (be displaced) to 10b light years in our lifetimes. The Hubble constant would have to shrink radically, or conversely, the velocity of recession would need to be stupendous, likely superluminal, i.e. v > c.

However, yes - if we observed a quasar say at 10b Ly that would amount to looking back in time 10b years.

The  attached diagram will also help give you an idea here.

The diagram denotes the "light cone" for the universe available to us at given times and how it expands.  Outside the light cone we cannot observe what's there (though we know for instance, that right now the actual radius of the cosmos is 46.5 billion LY, or much greater than the 13.7 billion light years we can observe) Note in the diagram that smaller expansion time corresponds to a smaller scale observable universe. Obviously, since the light cone available can go no faster than c.  


All Answers

Answers by Expert:

Ask Experts


Philip Stahl


I have more than forty years of experience in Astronomy, specifically solar and space physics. My specialties include the physics of solar flares, sunspots, including their effects on Earth and statistics pertaining to sunspot morphology and flare geo-effectiveness.


Astronomy: Worked at university observatory in college, doing astrographic measurements. Developed first ever astronomy curriculum for secondary schools in Caribbean. Gave workshops in astrophysics and astronomical measurements at Harry Bayley Observatory, Barbados. M.Phil. degree in Physics/Solar Physics and more than twenty years as researcher with discovery of SID flares. Developed of first ever consistent magnetic arcade model for solar flares incorporating energy dissipation and accumulation. Develop first ever loop solar flare model using double layers and incorporating cavity resonators.

American Astronomical Society (Solar Physics and Dynamical Astronomy divisions), American Mathematical Society, American Geophysical Union.

Solar Physics (journal), The Journal of the Royal Astronomical Society of Canada, The Proceedings of the Meudon Solar Flare Workshop (1986), The Proceedings of the Caribbean Physics Conference (1985). Books: 'Selected Analyses in Solar Flare Plasma Dynamics', 'Physics Notes for Advanced Level'. 'Astronomy and Astrophysics: Notes, Problems and Solutions'.

B.A. Astronomy, M. Phil. Physics

Awards and Honors
American Astronomical Society Studentship Award (1984), Barbados Government Award for Solar Research (1980), Barbados Astronomical Society Award for Service as Journal Editor (1977-91)

Past/Present Clients
Caribbean Examinations Council, Barbados Astronomical Society, Trinidad & Tobago Astronomical Society.

©2017 All rights reserved.