Question: > 
> He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power. After making purification for sins, he sat down at the right hand of the Majesty on high (Hebrews 1:3 ESV)
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I am interested in the phrase ‘he upholds the universe’
My question is:- How could Christ uphold the universe as a helpless babe and also after he had died before the resurrection.

Answer: 
His Son has been appointed heir of all things. That being the case He upholds, carrying on, prevents it from falling, sinking,
By the word, #4487

Strong's Concordance
rhéma: a word, by impl. a matter
Original Word: ῥῆμα, ατος, τό
Part of Speech: Noun, Neuter
Transliteration: rhéma
Phonetic Spelling: (hray'-mah)
Definition: a word, by implication a matter
Usage: a thing spoken, (a) a word or saying of any kind, as command, report, promise, (b) a thing, matter, business

The whole operation of the universe is in his hands. The entrance and presence of sin in the universe is all accounted for by one word… He has made a cleansing of sins.

Upholds:

To hold aloft; raise.
To prevent from falling or sinking; support.
To maintain or affirm: synonym: support.

He is holding and maintaining and propelling all things [the entire physical and spiritual universe] by His powerful word [carrying the universe along to its predetermined goal]. When He [Himself and no other] had [by offering Himself on the cross as a sacrifice for sin] accomplished purification from sins and established our freedom from guilt, He sat down [revealing His completed work] at the right hand of the Majesty on high [revealing His Divine authority],

It is because that He completed the work of making a cleansing of sins and therefore is seated at the right hand of the majesty.

That is how he keeps the universe going and upholding by His declaration of His work being finished because he sat down at the right hand of Majesty on high.
It is impossible to answer this question as we are not told so we cannot know. Indeed, if we knew the answer to this question, we would be God.

Equally mysterious, is the the fact that

* twice in the Gospel of John, we are told the Jesus raised Himself from the dead, John 2:19-21 and 10:17, 18.
* However, we are also told that the Father raised Jesus from the dead as well, Rom 6:4, Gal 1:1, Eph 1:17-20.
* Further, we are also told that the Holy Spirit is the source of life as well, Rom 1:4 & 8:11, 1 Peter 3:18.
* Acts 2:24, 3:15, 4:10, 5:30, 10:40, 13:30, 17:31, Rom 4:24, 1 Cor 15:15, Col 2:20, Heb 13:20, 1 Peter 1:3, 1 Thess 1:10 simply say that “God” raised Jesus without specifying any specific member of the Godhead.

All this simply illustrates the fact that the Godhead acts in complete unison. Just how that occurs we are not told. The same is presumably true of God upholding the universe.