‘’THE OBSERVABLE UNIVERSE’’

THE UNIVERSE MAY NOT HAVE BOUNDARIES.IT COULD BE INFINITE.THE PART OF IT THAT SCIENTISTS HAVE KNOWLEDGE IS BOUNDED AND FINITE.CALLED THE OBSERVABLE UNIVERSE,IT IS THE SPHERICAL REGION AROUND EARTH FROM WHICH LIGHT HAS HAD TIME TO REACH EARTH SINCE THE UNIVERSE BEGAN.THE BOUNDARY THAT SEPARATES THIS REGION FROM THE REST OF THE UNIVERSE IS CALLED THE COSMIC LIGHT HORIZON.THE LIGHT REACHING THE EARTH FROM AN OBJECT VERY CLOSE TO THIS HORIZON MUST HAVE BEEN TRAVELING FOR MOST OF THE AGE OF THE UNIVERSE,THAT IS APPROXIMATELY 13.7 BILLION YEARS.THIS LIGHT MUST HAVE TRAVELED A DISTANCE OF AROUND 13.7 BILLION LI GHT-YEARS TO REACH EARTH.SUCH A DISTANCE IS DEFINED AS A’’LOOKBACK’’ OR’’LIGHT-TRAVEL-TIME’’ ,THE DISTANCE BETWEEN THE EARTH AND THE DISTANT OBJECT.HOWEVER THE TRUE DISTANCE IS MUCH GREATER,BECAUSE SINCE THE LIGHT ARRIVING AT EARTH LEFT THE OBJECT,THE OBJECT HAS BEEN CARRIED FARTHER AWAY BY THE EXPANSION OF THE UNIVERSE.[REF:page22,UNIVERSE;MARTIN REES]{critical comments}.