Any echo of the big bang would be coming from all directions and going in all directions. It did not leave from here and come back, it left from some other point in the universe and is just reaching us.
That's how the current estimation of the universe age was made (only it was about the background space temperature of about 4 Kelvin). This time, by measuring magnetic waves they seek to find a (even slight) difference in expanding directions.
/not-a-scientist