Jamieson, Fausset and Brown on Hebrews 1:2

2. in these last days--In the oldest manuscripts the Greek is. "At the last part of these days." The Rabbins divided the whole of time into "this age," or "world," and "the age to come" (He 2:5; 6:5). The days of Messiah were the transition period or "last part of these days" (in contrast to "in times past"), the close of the existing dispensation, and beginning of the final dispensation of which Christ's second coming shall be the crowning consummation.

by his Son--Greek, "IN (His) Son" (Joh 14:10). The true "Prophet" of God. "His majesty is set forth: (1) Absolutely by the very name "Son," and by three glorious predicates, "whom He hath appointed," "by whom He made the worlds," "who sat down on the right hand of the Majesty on high;" thus His course is described from the beginning of all things till he reached the goal (He 1:2, 3). (2) Relatively, in comparison with the angels, He 1:4; the confirmation of this follows, and the very name "Son" is proved at He 1:5; the "heirship," He 1:6-9; the "making the worlds," He 1:10-12; the "sitting at the right hand" of God, He 1:13, 14." His being made heir follows His sonship, and preceded His making the worlds (Pr 8:22, 23; Ep 3:11). As the first begotten, He is heir of the universe (He 1:6), which He made instrumentally, He 11:3, where "by the Word of God" answers to "by whom"' (the Son of God) here (Joh 1:3). Christ was "appointed" (in God's eternal counsel) to creation as an office; and the universe so created was assigned to Him as a kingdom. He is "heir of all things" by right of creation, and especially by right of redemption. The promise to Abraham that he should be heir of the world had its fulfilment, and will have it still more fully, in Christ (Ro 4:13; Ga 3:16; 4:7).

worlds--the inferior and the superior worlds (Col 1:16). Literally, "ages" with all things and persons belonging to them; the universe, including all space and ages of time, and all material and spiritual existences. The Greek implies, He not only appointed His Son heir of all things before creation, but He also (better than "also He") made by Him the worlds.