When you look at a star in the sky you may be “seeing” what that star looked like millions of years ago if that star happens to be millions of light-years from earth. Similarly, these scientists are “looking” in areas of the sky (the universe) thought to be furthest from earth. Theat’s how they “peer back in time”.
What I’ve always wondered is, if scientists could “peer back in time” to one nanosecond into the existence of the universe and discovered a mirrored surface which was perfectly aligned toward the scientists, what would that mirror be reflecting?
Well, considering the reflected light would take just as long to return, you probably wouldn't see it if it were at that point. The light reflected from us would have to reach the mirror, then reflect back, doubling the time frame of the distance you are looking at. One of those fun space-time things to contemplate but only a few in the world can calculate (I wouldn't even pretend to try..)