( ( ( ( ( ( ( The ( light ( travel time ) ) ) ( for ( the ( most ( distant ( visible galaxies ) ) ) ) ) ) ( is ( ( ( ( several billion ) years ) , ) ( comparable ( to ( ( the age ) ( of ( the universe ) ) ) ) ) ) ) ) , ) so ) ( ( by ( studying ( distant galaxies ) ) ) ( we ( ( are also ) ( ( looking back ) ( in time ) ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (JJ light) (NN travel) (NN time)) (PP (IN for) (NP (DT the) (RBS most) (JJ distant) (NN visible) (NNS galaxies)))) (VP (VBZ is) (NP (NP (QP (JJ several) (CD billion)) (NNS years)) (, ,) (ADJP (JJ comparable) (PP (TO to) (NP (NP (DT the) (NN age)) (PP (IN of) (NP (DT the) (NN universe))))))))) (, ,) (IN so) (S (PP (IN by) (S (VP (VBG studying) (NP (JJ distant) (NNS galaxies))))) (NP (PRP we)) (VP (VBP are) (ADVP (RB also)) (VP (VBG looking) (ADVP (RB back)) (PP (IN in) (NP (NN time)))))) (. .)))
The light travel time for the most distant visible galaxies is several billion years, comparable to the age of the universe, so by studying distant galaxies we are also looking back in time.
(ROOT (S (PP (JJ Due) (TO to) (NP (NP (DT the) (NN time)) (SBAR (S (NP (PRP it)) (VP (VBZ takes) (PP (IN for) (NP (NN light))) (S (VP (TO to) (VP (VB reach) (NP (PRP us)))))))))) (, ,) (NP (NP (NNS galaxies)) (PP (IN at) (NP (NP (DT the) (NN edge)) (PP (IN of) (NP (DT the) (NN universe)))))) (VP (VBP are) (VP (VBG distant) (PP (IN in) (NP (NP (NNS terms)) (PP (IN of) (NP (NN space) (CC and) (NN time))))))) (. .)))